The reason Fastmail's feature matters is specifically because it's not using the + trick or a catch-all domain. They're 'real' aliases, not just Regex filters or wildcards.

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.