> This is just how the web works, and there is no easy around it without losing features people care about [...]
Well, apart from using a separate email address for every single "provider"?
(Spoiler: there's no way I'm going to sign into your service with a shared email ... you get <youservice>@<me>.com)
Some services only allow signups from the big free providers like gmail/outlook/etc. because those providers are doing more consistent KYC and anti-spam measures than anyone else by far, and unfortunately it does cut down on the amount of spam by a lot. For most people nowadays you cannot even create a new gmail account without directly linking it to a mobile phone.