Research on legacy Mojang accounts

It is already widely known that unmigrated accounts can no longer be used to log into official launcher, and there is almost nothing you can do with them on website, except migrate. However, some third-party launchers, such as PolyMC, still offer the option to log into the game from Mojang account, even though it is not possible to play multiplayer in such case. This raised the question in my mind: how much exactly can still be done with unmigrated account?