This would've not impacted you, if you did move - they stop offering the feature to new customers, but will keep it working for existing users.
I've had a couple of free emails with custom domains setup probably 15 years ago (Microsoft went through a few different names for this service since) and they work to this day. The only difference is I can't create new email accounts in the custom domains.
It would, because I make aliases and other mailboxes occasionally. (E.g.: Apple/Google developer program public-facing alias) I don't want to be locked into just the one name forever.
I've had a couple of free emails with custom domains setup probably 15 years ago (Microsoft went through a few different names for this service since) and they work to this day. The only difference is I can't create new email accounts in the custom domains.