I couldn't speak about "coding twitter" in general, but Apple developer Twitter in specific has almost entirely migrated to Mastodon (as well as a large contingent of the Apple news media). I see a lot of non-Apple devs there too.
You: I think this is a biased view held by people who migrated to Mastodon.
Me: ???
I don't know about Apple Twitter myself, but Web Dev Twitter seems more active. Infosec Twitter was a notable niche that loudly proclaimed they were moving to Mastodon but in fact many of them are still quite active on Twitter.
But they didn’t when Facebook launched Poke, did they? You’d be forgiven for forgetting it ever existed:
https://www.theverge.com/2014/5/9/5700732/facebook-poke-is-d...
Point is, FB actually doesn’t have a great record of launching brand new apps. Instagram Stories worked by pivoting almost the entire app around them. Remains to be seen if Threads fares better.
Coding maybe, but I think Musk's perceived hostility towards the trans community is going to drive medical and educational Twitter to the first viable alternative, and they will serve as catalysts to peel off other professional groups.
This is regional/network-effect based.
I've seen it situated the exact opposite opposite direction in some locales.
I say this as someone who is no big fan of Meta.