I do this using a catchall account with Fastmail, but I don't even bother obfuscating the vendor name. Occasionally they don't let you use an email with their name in it, which which case I add 'isdumb' to their domain and then use it.

It happens occasionally that they get confused, but so far I haven't run into any issues.

I do agree in principal that it would be better to have some sort of hash that maps their domain to the e-mail in a way that is easy for me to know but hard for someone else to figure out. Ideally in a way that is still reasonably easy to pronounce over the phone.

What would be really slick is to have such functionality integrated into a password manager.