The standard (as in most common setup) for djb's qmail, was to accept/forward mail for <prefix>@user.example.com to user@example.com, as in: debian-list@e12e.example.com rather than debian-list+e12e@example.com.

While I believe the + was common for postfix, exim and sendmail?

There even was a spam-fighting scheme that used this - it took a key, and optional date, and a sender address - and generated a unique address you could give out, that was only valid for a certain time, for a certain set of senders.

If the system received a mail with a from address that didn't match the cryptologically signed to-address (e12e-xjjgff65477fc@example.com) - the mail was held back, and the system generated a reply, with a signed reply-to address. A sort of manual grey-listing.

The point is that foo+bar is a convention that many providers follow and not an actual rule of email, and therefore blocking that doesn't make sense because you'll block legitimate addresses too.

Similarly, mail providers can come up with all sorts of different conventions if they want. For example, when setting up a new domain in FastMail, it offers the ability to accept anything@user.dom.ain and turn that into user+anything@dom.ain, and it offers user@anything.dom.ain which it will deliver to user@dom.ain. So here we already have two new conventions that sites can't possibly detect as alternatives to the normal foo+bar@dom.ain.

Depending on your email provider, there are a ton of different ways to create brand new email addresses. Blocking off the single pattern of foo+bar@example.com just punishes legitimate users without causing any problem for spammers. Besides the fact that foo+bar@example.com may in fact be a legitimate unique email address, there's also just the case of people who routinely use username+sitename@example.com when signing up for anything as a way of tracking where incoming email is coming from.