On both social media I still use, I followed bunch of indie ttrpg devs. I had to unfollow the ones on tumblr because they don't talk about anything but how much they hate D&D, up to including writing weird headcanons about people who help popularize it (I actually seen someone on tumblr claim Brenann Lee Mulligan must HATE D&D because only dumb people could like it and is only pretending to like it for clout) or bich about how D&D fans only want to play D&D and get mad when you tell them they're complaining about a small minority.
meanwhile indie ttrpg devs on Bluesky constantly promote their work and inform of sales of their products. I found many cool games through bluesky, meanwhile nove through tumblr.
Why do the tumbler users not simply build a third party alternative to migrate to?
Well there's wafrn, but I think tumblr is a bit of a special case because so many people find it hard to understand even compared to reddit or twitter, and due to how user-focused and offkey its content is it's kind of hard to migrate to a different platform - with lemmy and mastodon, you can search a bunch of keywords and quickly have a feed with content that mostly interests you that you can build from, but this wouldn't quite work for tumblr-style platforms. On top of that, lemmy and mastodon users tend to be focused on techy and political content, which is a different demographic from tumblr's.
they tried with Pillowfort and it crashed and burned