Hey there, as of the 25th of March, players are unable to unlink platform accounts from a Bungie.net Account. The problem here is that both your XBox and Steam have their own Bungie Accounts that they are already attached to. The situation people are more likely to run into, is say someone has a Steam and an XBox pairing; if they wanted to unlink Steam from it, they can no longer do this. But in your case by trying to link Steam to XBox, you're trying to unlink Steam from its existing Bungie Account, and that's another one of the things the 25th of March change prevents. It is not anything to do with being a migrated battle.net account, the error message "Platform accounts cannot be linked to multiple Bungie profiles" is saying that Steam account is already connected to a Bungie Account which it can no longer be unlinked from, and so can't be linked to another.