Some websites do that. They block at the domain level. It tends to become a escalating game of spinning up new alias domains.

https://proton.me/support/creating-aliases#additional

If you have a paid plan, you can create at least 10 additional addresses (depending on your plan) with any Proton domain (@proton.me, @protonmail.com, @pm.me, or @protonmail.ch) or a custom domain, if you have one.

I have a custom domain, which websites never block, because they have no way of knowing that I use it for aliases.