I definitely vividly remember needing it a few years ago, but right now I can try to sign up and it says "Mobile Number (optional)" (Maybe that's based on some security heuristics).
No doubt they're letting me through because some security heuristic says I'm a real human, and I'm sure they'd eventually make me provide a number if I continued using the account (this happened to me with my university G Suite account a couple years ago and I needed to contact my IT department to manually disable the phone challenge), but so far I can't see any evidence that they're doing anything unreasonable.
Perhaps they're requiring you to use a number because you've tested it a lot.