Similar to the requests for 'wildcard support for spamgourmet' and 'searches using the "+" syntax' Fastmail (and I suspect other providers) offer the facility to send email to <anything>@<myemail>.fastmail.com - where the "normal" email address is firstname.lastname@example.org
I use this extensively to register unique email addresses for each site (so if spam comes in i can see where it was leaked from) but in many cases i've no record of which sites i've used addresses on.
as such it would be very useful to check for *@<myemail>.<providerdomain.com>
to prevent abuse e.g. someone trying to register *@hotmail.com then send a verification email to <randomstring>@<myemail>.<providerdomain.com> - as the wildcard should catch EVERYTHING it should receive email sent to a random string and if you don't notify what the random string is then there is no way i can see of someone gaming the system and registering a domain for which they don't have wildcard access.
The big difference with the plus aliasing syntax is that it’s a very broadly adopted pattern that whilst not a spec (and frankly, that’s a big part of why this feature doesn’t exist), is broadly supported. I don’t want to get into a cycle where one specific mail provider (and a smaller one at that) implements something specific to them and HIBP needs to implement that pattern.