With some you can do the + trick (which gmail probably still does) but i just have my domain as catchall and it works pretty great with blacklisting.
If you're using the + trick, you haven't gained any privacy, because I can strip the + and get your original address.
If you're using a catch-all domain, you haven't gained any privacy, because the domain remains a unique identifier for your all of your accounts. It's good for organizing, but not for privacy, because you're still publicly attaching your identity to every email you send.
With fastmail, I don't need to do [email protected] or [email protected]. I can just do [email protected]. That's a really large privacy win, since it gets rid of one of the biggest and least regulated unique identifiers that services can share with each other.
I don't know if other providers like Outlook are also offering 'real' aliases. I'm happy if they are, I think this should be an industry standard feature. Either way, switching to any provider does will be a pretty significant feature upgrade over Gmail, even if you're currently using a paid Gmail account with your own domain.
So It so different from random domain catchall?
The reason why i would be worried about Fastmail is that they have are Australian company with servers in US. Both of those mean that Law enforcement can simply ask for users emails.
Now i am for sure not target of Law enforcement or goverment so i dont care but i am not sure why i wouldnt use service thats in better juristiction and is privacy focused.
With every account you get a finite number of aliases you can create, but in practice that number is high enough that I just use a new alias for every site I visit.
Unlike in Gmail, these aliases don't contain any references to your original address. So if you're signing up for a dogwalking service, you can create an alias for `[email protected]`, and then if you start getting spam to that address, you know where it came from, you know that there's no chance your real address will be reverse-engineered from your alias, and you can auto-reject or sort everything to that address into a separate folder without affecting any of your other emails.
I have separate email aliases I distribute to friends and family members so that if I ever run into a doxing situation or for some reason need to go nuclear on my email, I can turn everything off except for them. I also have my email linked to my own domain of course, but when I sign up for most commercial services, I use @fastmail.com aliases. That way I know that there's no way for those services to track me across accounts/websites via my personal domain name.
And everything gets organized in the same inbox, same account. I consider it to be a killer feature.