This might depend on the country you're in, but I'm quite certain I've gotten locked out of the signup flow in the past when I refused to provide a phone number.
I just tried it from my Android phone (GrapheneOS) and it still asks to verify a phone number when trying to create an account via a web browser. (Strangely, even though it's a private browser session it just asks to confirm my number by sending an SMS, not asking me for my phone number like it does on desktop -- I wonder how that works...)
If you're saying that the account creation flow through the system accounts application doesn't require a phone number, how are you sure that Google doesn't just collect the phone number directly from your device (they could even silently verify it through a class-0 silent SMS)?
Does it also not ask for a phone number if you factory reset, remove the SIM card, and do not register the phone with a Google account? Maybe they track the IMEI instead?
