Wearing clothes of the opposite gender.

Being topless regardless of gender in public during hot days.

I dunno about taboo, but I think there'll be a lot fewer "monosexual" (homo/hetero people) and a lot more bi/pan people. I think we're seeing an increase already in acceptance that most people have at least a few people of their non-preferred gender they're attracted to, and those kinds of mentalities will permeate to a mushy continuum of sexuality, rather than hard categories.

Drugs. The prohibition of psychedelic substances in particular is looking more like a crime against humanity since we are rediscovering their therapeutic properties in the west (that shaman have known for mellenia).

Discussion on the topic of mental health. Virtually nothing was known about mental health until very recently. We are the first generation that even talks about it. Therapy didn't exist in any practical and organised sense for my mother's generation. If you got PTSD during WWI, it was a death sentence because your own frigging side would shoot you.

Free-the-nipple hopefully.

I know there's a lot of humor over this campaign but the fact that it is illegal for one gender to do something and not the other gender and the length that media and social go to to censor only female nipples is kind of mindblowing.

Sounds like a simple and easy thing that will eventually pass into absurdity but with the whole "save the children" crackdown going on, I'm not optimistic that this is a freedom women will enjoy in a 100 years.

Polyamory

Access to abortions, I hope, sh!t is getting long.

Human genetic modifications for improved physical/mental/emotional performance and aesthetics reasons. I'm sure furries will get what they want someday.

Being overweight. It's a matter of years before a magic pill cures obesity.

Obesity will no longer be seen as a social taboo, but as a disease than can be cured.

Voluntarily chopping off your arm. (To replace it with something else)

I suspect that some degree of polyamory may be more socially acceptable in 100 years than it is now.

Don't worry too much, Kojima will make a game about it eventually.

I am going to operate under the following assumptions:
-the current global trend towards authoritarian governments will continue and become more prevalent
-balkanization will be the new norm: an atlas will show more numerous and smaller countries
-climate change (extreme heat, extreme humidity and sea level rise) will make large regions functionally unfit for human habitation by reasons of lethal heat and/or humidity, loss of coastal access, lack of potable water and/or loss of sustainable agriculture.
-we'll be well into the technological curve for AI and robotics. We'll have gone past the early stage where people over-estimate technological capabilities and far into the later stages where people will under-estimate technological capabilities
-if cash is still legal, it will be useless for all legitimate transactions because no institution wants it. If it still exists, it will only be useful for peer-to-peer illegitimate transactions: crime, drugs and sex.
-whatever is bad now will be worse

So: social taboos that exist today that will not be taboo in 100 years?
-slavery: we already see slavery in all but name in the form of privatized prisons and wage-slavery (work a soul-killing minimum wage job, or die/be homeless). What if the cost of being able to emigrate from a country or region that is uninhabitable is slavery, whether real or de facto? It's the cheapest form of labor.
-murder: being deemed outlaw will make a comeback. An outlaw is outside the protection of the law, so killing an outlaw is not a crime.
-extortion: governments and government proxies (militias, death squads, religious sects) will exercise sanctioned extortion
-hoarding: if you are living in an unstable balkan state or are an unpopular minority in one, hoarding will not be pathologic
-civilian ownership of firearms
-racism and nationalism; best way to keep out undesirable climate refugees is to de-humanize them
-corporations being into every piece of the pie: a logical extension of the trend to privatization or "wanting government to be run like a business" is the replacement of nation-states by corporations or zaibatsu-like alliances of multiple corporations

vegan diets will be more prevalent, it's more environmentally & economically sufficient, and lab grown meat will eventually be held to higher standards & be more widely available than the farming industry

Eating meat. I think two things will happen: (1) we'll discover that there's very little difference in feelings between humans and other animals and (2) science will allow us to produce meat in a lab.

I'd be surprised if in 100 years there's not at least one place in the world where wearing a pet collar is considered socially acceptable.

So…. what book were you reading?