I found out two weeks ago after removing it because I thought it was no longer needed after I switched away from SMS-based 2FA. No warning or anything at all in the UI that indicated the removal would result in a suspension, and they don't even let you go back to your settings to re-add it!
My guess is that they have some kind of scoring system and having a phone number increases your "trust" score. When you removed it your trust score went below some threshold.