Already in the past I've been (ab)using Fastmail's email alias feature for this kind of purpose. Though it was a bit inconvenient as the UI always said that it takes 15 minutes for the new alias to become fully active. Great to see they now have simplified this!
When I get spam to a particular alias, I blacklist it to my spam folder. Naturally this might get a little difficult if a spam bot ever figures this out where I'd need to move to a whitelist rather than blacklist process. But it's worked flawlessly for years so far.
At least with my method I don't need to create the alias in advance.