I decided to give Mastodon another shot here recently, I found exactly one promising leftist instance, mastodon.lol. Bills itself as an "LGBT-friendly antifascist mastodon instance"
I watched another user get banned for saying that Azov were neonazis and I got banned for sticking up for them, then as part of his 240V power trip the admin posted a pity thread where he claimed we called him literally Hitler.
The only other leftist instance I saw welcomed all socialists except "authoritarians" so I assume it's either a socdem or sectarian anarchist instance.
TL;DR - Mastodon hasn't changed at all, it's still ran by power-tripping micro tyrants
Actually creating and running one would be pretty easy but I wouldn't want to be responsible for moderating it. I modded one of the sub's ofshoot Discord channels for a while and that ended up being a nightmare