https://api.mojang.com/users/profiles/minecraft/<name> used to return legacy: true if the account was unmigrated, as shown on https://wiki.vg/Mojang_API#Username_-.3E_UUID_at_time

Was this changed to another API, if not, can this be reimplemented?

This was used on community support for knowning when we should redirect users to the migrate page instead of the normal steps, or if we noticed the account was migrated when the user didn't do it themselves we can redirect them to Mojang support, or when a user asks if they should login with email or username