I miss the "Tales from..." subs. Tales from tech support was regular reading material for me for many years, and in general just having a place to commiserate with others in the same field as you is wonderful. The other ones also helped me be more concious of what I could do to keep myself from being a nuisance to other professionals like my doctor and pharmacist.

More niche, I miss the gunpla sub a lot. We have subs for model making and tabletop miniatures, but the gunpla community was very well run.

In general, I think the lack of moderation tools has made it difficult for communities to do regular "event" posts and the like which used to really help keep subs alive, guide discussions, and gave good examples of the type of content that fit. Like it's a lot easier to start a new conversation at a party where everyone is talking than to be the first person to speak up in a silent room.