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).

Yeah and it also only works on your phone (or if you know how to make Google think you are on your phone) and in certain countries. All to my knowledge and based on my tests.

I thought the same but I just tried on firefox desktop (Windows) and spun up a new google account with email, password, fake first+last name and fake bday. Really, I was expecting to be stopped at "Phone Number required" but it is indeed optional.

I just did it from Firefox on Linux in a private tab near Washington, D.C.. Fake name, no phone, no backup email. I was able to log out, sign back in, and send an email without any trouble.

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.

We are talking about creating an new account, not about signing in.