How useful is this, though, for just a username?

I would think the purpose here would be that you would use unique aliases per service to limit your own risk in the event of a site breach. However, the vast majority of websites these days require a username and an email address. In which case, if I've got 50 unique usernames but they're all tied to a single email, how much am I really protecting myself if the email address gets included in the breach?

One thing I do, which no clue how helpful it really is, is use a custom domain for my emails with support for catch-all addresses. When I sign up for a new site, I typically put my email as something like "<site-name>@<my-domain>.com".

If my data were exposed, I guess someone who realized that could try any variation of the site's name to figure out the exact one in my address, but you could always do something more unique than that. Even something like generate a BitWarden password and use it as the user for the domain.

It's less a protection schema and more a canary schema. I use plus-addressing for basically every online account that I create that does not need to be professional in nature. The benefit is that if I start receiving emails from Foo.com sent to [email protected], I know that my bar.com account got compromised and I can do something about it to limit my exposure.

I've found that a lot of websites don't like "+" or even intentionally detect it to reject it or subvert the user's interest.

I think a good system would be a randomly generated handle like [email protected] where my real email probably isn't [email protected], but the number is different for each service.

I also use random email aliases for my domain. The random username is because I don't want people to be able to search my username for a service and link me to other accounts online.

One very slight benefit I see is (better) standardization of username generation. If everybody has their own method of generating usernames (eg long string of numbers vs 2 random words), then it's possible to differentiate usernames based on the style. Now that generation is automated via lastpass or bitwarden, there are two standard styles and makes it harder to fingerprint users based on it

I agree on uniqueness. I think you really want at least 40 bits of entropy (so no expected conflicts up until about a million usernames).