There is one surprisingly good feature in yahoo mail that I wish google would provide: the ability (requirement, actually) to create a custom prefix for disposable email addresses. Just pick something completely unrelated to your real username. You can only create it once and it is forever your prefix, so don't mess up this step! Also, unlike gmail, you have to register a suffix when you want a new address in the form of prefix-suffix@yahoo.com. But this has the added benefit that you can delete any of the suffixes from the list, thereby stopping incoming messages from that address. You can have up to 500 active suffixes.

Finally, since the format uses a dash rather than +, I have never once had a disposable yahoo address rejected.

Not sure if that made any sense, but here's a link to the docs:

https://help.yahoo.com/kb/SLN28338.html

Why would they care enough to do so?

Yes, this is part of standard data cleaning practices. You always remove whatever custom formats a provider allows to get the raw minimum email address (this is only for export, you still send official communication with a user to their original specified address).