From what I’ve seen so far (although I’ve had all my email accounts for a good long while now) is that they don’t necessarily request a number but they will request some sort of contact or payment info.
As Andy Yen admitted in a podcast with the Linux Experiment email is rife with abuse and anything that makes it too easy to make an account will be targeted by scammers and the like and eventually get blocked by (at minimum) all the big-tech email providers, altogether making their service worthless. So they have to get something that can be used to prevent abuse.
I originally got away creating an iCloud account with a gift card, although not technically free and someone could probably trace that back to me with enough effort. I think it’s limited to 1GB and just using email if you don’t connect it to an Apple device. Not sure if the same rules still apply though. It’s also a sunk cost if it doesn’t work anymore.
My Vivaldi (the browser) account has email now? Which I’m fairly certain only required registering with an email address. But I have no idea how I qualified or if it’ll last. The Vivaldi browser itself seems to operate it as a desktop IMAP client.