If you try to register multiple Gmail accounts from the same location (such as a public library), at some point Gmail will start to ask you for a phone number verification.

This is something I saw a few years ago in France (before GDPR), there is no reason it has changed.

I suspect it is to avoid fraudulent accounts while still letting a few mails being registered without verification for first time users in a location.

What was the last time you signed up for a Gmail account? At least in The Netherlands they've been requiring mobile phone numbers for years. Hell, I have a dummy account (Maps locks a lot of functions behind login) and I can't log in to it anymore since Google wants me to register my phone number 'for security and validation purposes'