They're supposed to be against minors on there, and unaddressed reports could be the justification.

actually, seems to be a widespread ban

Ever since the API crackdown thing especially, they've going through and quietly banning all kinds of subs that might scare advertisers, or that they just don't like for political reasons. The venn diagram for those two things has a lot of overlap of course. They've been killing off small leftist subs (as we know, they killed off the bigger ones a while back now), piracy subs, porn subs (no tears shed from me about that, though I expect it will probably end up hurting sex workers too like that sort of thing always does) etc. I think r/piracy is so big that there would be enough backlash which makes reddit reluctant to axe it, but I expect it will be on the block too before much longer. But for example, r/fmovies was banned last month.

Anyway, I expect the ban on r/TransDIY is that part of the venn diagram that it scares advertisers and is something reddit is politically in opposition to. Like What_Religion_R_They said, the reasoning and justification they'll use is bullshit about it being potentially harmful to minors.