Quite a few of us migrated to ms accounts, only to get blocked and the minecraft username/account ransomed until we gave Microsoft our mobile phone numbers due to "suspicious activity".
Said activity being signing in to the launcher post migration....
I refuse to migrate my account, until the day they shut the Mojang servers down. Which I fear may be soon.
Does PolyMC still let you log in with a Mojang account? I know that MultiMC doesn’t.
For multimc there's a fork called ultimmc that allows playing without an account.
Some servers do not require authenticated accounts and work in so-called "offline mode" where authenticity of usernames is not verified with mojang servers (created for playing on a LAN). Many servers on the other hand require usernames of connecting clients to be verified -- and this does not work anymore (at least for me) with the mojang login. wiki.vg has a large repository of minecraft protocol descriptions: [1] and [2]
[0] http://ni.sijanec.eu/sijanec/n/tree/etc/portage/patches/game... [1] https://wiki.vg/Legacy_Mojang_Authentication [2] https://wiki.vg/Protocol_Encryption#Authentication