I setup a sequence of letters which makes it completely unobvious for anyone who doesn't know the sequence

If you own your own domain, it's trivial for them to notice that it's a vanity domain for a single user or small group. The only way to hide is to have thousands of friends with diverse interests using your domain.

Trivial to a human looking at the data set, but it's something the algorithm would have to be trained to recognize.

Not really - in practice the tracking system can have a list of email providers and treat every other domain as one user (especially ones with catch-all setups).

I don't think it's trivial at all? I sign up with random first names and last names, my email address doesn't have the actual company name and I don't think there's that many people with vanity domains to justify adding code to track this use case.

For historical reasons I use a dash. This works just fine 99+% of the time, but very rarely a site will not allow a dash in emails.

Thank you - finally someone who knows that it was always a dash.

What do you mean? What was "it"? Subaddressing semantics has always been at the discretion of the email service provider.

As far as I know, subaddressing was invented as part of qmail, which used a dash: https://en.wikipedia.org/wiki/Qmail#Innovations

Fastmail supports subdomain addressing, such that instead of "[email protected]" you would use "[email protected]", which will forward the message to "[email protected]" and even file it into a folder for "service".

You can also create free aliases on a completely different domain, and I think they have a plugin with 1Password to do this for each login you create.