Gmail didn't mandate phone number the last time I created dummy account (~1 month ago). I don't have phone number linked even with my primary account.

I don't doubt that. In my experience sometimes a phone number is required and sometimes not. When it prompted for a phone I didn't find a way to work around that.

I find that when I create a dummy account from a clean browser (no cookies) with a VPN, a phone number is required. I wouldn't be surprised if they do some internal risk/dodginess assessment based on several factors.

Can you then remove it after the fact? I was able to remove the phone number from my account without it complaining.

They do sometimes though depending on how "risky" they deem you based on a secret criteria. I've never been able to bypass their phone number requirements.