I've had services refuse my fastmail.fm email address, with the reason that they don't allow "disposable" email accounts. But they accepted my gmail.com address....

Not that this was their criteria (they seldom if ever think it through to this level), but gmail requires phone number after a certain point. And they only allow 4 accounts per phone number.

That's a good concern.

I can't fully speak for the Fastmail folks, but I know that there are a few upper limits for how many masked email addresses that one account can create. We tried to set them unreasonably high to allow for all manner of legitimate use while still preventing bad actors. They're also monitoring usage and tuning that limit. Plus, you can always email support and ask for a increase for your specific account, if you ever bump up against it.

Sell private domains as an "enterprise" feature, and have different sets of IP blocks warmed and ready to go for when they eventually get blacklisted. But selling it as a service involves a higher level of effort due to that exposure. Configuring a private domain for just yourself to solve the problem just for you doesn't have the same risk exposure.

mailinator's been around providing this (as a recieve only) service for decades by this point.