I do this too, with Fastmail, but they do have a limit on how many aliases you can have without paying them more (last I checked).

The way around this is that they let you do alias+something@domain.com, or for sites that won't accept a plus address, alias@something.domain.com. This works very well for me, although I get funny looks when some vendor asks me for an email address and I give them e.g. alias@your_company.domain.com. Especially fun when you end up having to spell it out while the clerk eyeballs you like you're trying to screw with them.

I started doing this thinking it would let me identify who was selling/leaking emails to spammers. I now have a lot of unique email addresses like this, one per vendor. Fastmail is either excellent at filtering spam without me ever seeing it, or vendors just aren't selling/leaking my address(es); the only 'vendor' that I'm sure distributed my address after 3+ years is a state government entity.