From the Julia Serano article I linked:

But the thing is, the word “transgenderism” began as an in-community term to refer to the phenomenon of transgender people (read: our existence and experiences).

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The word transgenderism has been around for as long as I have been aware of transgender activism. It appeared in the titles of explicitly trans activist books such as Patrick Califia’s 1997 book Sex Changes: The Politics of Transgenderism, and the 2003 anthology Bisexuality and Transgenderism: InterSEXions of the Others. It appears in Kate Bornstein’s Gender Outlaw, Leslie Feinberg’s Trans Liberation, and countless other trans activist books, including Whipping Girl — most notably in the chapter “Coming to Terms with Transgenderism and Transsexuality.”

In all of these cases, the word “transgenderism” was used in a neutral manner to denote one of two things: the phenomenon of transgender people (our existence and our experiences), or the state of being transgender (e.g., I might talk about my own transgenderism). It is very common in English to use the suffixes “-ity” and “-ism” to create nouns that describe a phenomenon or state of being — for example, I might talk about my curiosity or intellectualism. So transsexuality and transgenderism are linguistically akin to those examples, and to me talking about my bisexuality, or discussing the subject of lesbianism more generally.

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But then, in the last couple years, some TERFs (trans-exclusive radical feminists) have purposefully misappropriated it in a way that confuses the state of being transgender with a potentially dangerous political ideology.

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This incorrect usage seems to purposefully capitalize on the fact that transgenderism is not an everyday word (so it will strike trans-unaware readers as somewhat alien) and seems intended to invoke certain oppressive ideologies (e.g., sexism, racism, fascism, and others) that also just so happen to end with the suffix “-ism.”

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The most common complaint in this campaign against transgenderism centers on statements like “transgender people are not an ‘ism’.” But as I said earlier, “isms” aren’t always ideologies — many of them (e.g., magnetism, metabolism, hypothyroidism, lesbianism, transgenderism) are simply naturally occurring phenomena. Plus, not all ideological “isms” are bad or dangerous — for instance, I personally think that feminism (as a whole) is a positive and beneficial thing.

OK, so with all that context, let me try to address your comments and questions:

This comment has been on my mind since seeing it yesterday, and I felt too foolish to ask, but is there an equivalent term without a dogwhistly connotation? 'Cause I’m pretty sure “transgenderism” has come out of my mouth before, and for the sake of describing “the state of being transgender” I can’t come up with anything else that suits.

It depends heavily on the context. In scientific journal articles, what I see is the discussion is centered on "gender dysphoria" - they don't talk in terms of "being trans" or "transgender" anything, it's always "gender dysphoria". I think this won't work in the community because a lot of people feel that centering dysphoria is gatekeeping and excluding trans people who don't feel dysphoria or only feel euphoria, etc.

It's not even clear to me exactly what "transgenderism" encompassed, I feel that binary trans women who medically transition end up getting a lot of the attention from media, and most mainstream narratives about trans people center binary trans women who medically transition. So when someone says "trans" or "transgender" I think most people think of a binary trans woman who medically transitioned.

This creates a lot of problems because the history of how those terms even came about was as an umbrella term to include more gender-diverse people than just "transsexuals" (i.e. trans people who medically transition; as an aside, I don't like that this term has become taboo, I think it's a succinct term to communicate what I mean and I think we should actively resuscitate the term and take it back from the pseudoscience gatekeeping transmedicalists who seem to have made such a stink on it).

"Trans" and "transgender" is supposed to include non-binary folks, genderqueer people, cross-dressers, drag performers, agender people, bigender people, etc. - it's meant to be a big tent term under which all of us can identify and cooperate against the common struggles we face in a society that punishes gender non-conformity.

OK, back to your comment:

“When I was a child, there was very little public discourse about transgenderism.”

In this sentence, I’m trying to convey a lack of discourse about transgender existence as a whole, encompassing not only the state of transgender experience (dysphoria, isolation, mental health impacts), the process gender transition, social and political stigmatisation, and so on.

If I were to substitute “transgenderism” with “being trans”, I feel it speaks more specifically to one’s internal experience; the state of being a certain way in a vacuum. Maybe that’s just my interpretation and something I need to change.

Let's try a few sentences out like you suggested:

“When I was a child, there was very little public discourse about transgenderism.”

Yes, at this point, the community will think this sounds right-wing and transphobic, and the average person on the street will think you're talking about how crazy the world has become because trans people exist now.

“When I was a child, there was very little public discourse about being trans.”

I agree this is a little awkward - "being trans" does feel a bit out of place.

But what about:

“When I was a child, there was very little public discourse about trans people.”

or even:

“When I was a child, there was very little public discourse about trans people and their experiences.”

I think what you might be missing is that we don't need to perfectly substitute "transgenderism" with a single word or phrase that means the same thing in all contexts. If we drop that requirement and just focus on communicating what we're trying to mean, I think the words will just come.

There are lots of different ways to convey that idea too, in a comfortable context with someone I know and trust, I might just bemoan:

Trans people didn't exist in the 90s!

Of course this wouldn't sound great outside the context (an anti-trans person would certainly say the same thing, but mean a very different thing), but the context allows for the meaning to flow the right direction and not imply the wrong thing (i.e. you're not saying trans people didn't actually exist, you're saying there was so much less awareness and tools for understanding trans experience then, and more trans people were stealth or in the closet as a result, less visible - but still there).

So I guess I just wouldn't get too hung up on it, it's just good to be aware of the additional anti-trans connotation "transgenderism" has gained so you don't accidentally signal you are anti-trans yourself, but I think language should be open and we should feel free to carve out meanings of our own in our own spaces and contexts as they support them. Maybe there will be ironic uses of "transgenderism" that will allow the term a foothold in our community again, maybe the term will find new life that way, or maybe not.

Either way, the point isn't to become rigidly scrupulous about our language and police ourselves and others, it's more about raising awareness of the meanings that words carry in different contexts.