Re: I've seen a definite uptick in these
The username+something@domain is called plus-addressing and has been available in sendmail (among others) since before gmail was a thing.
Don't try claiming that it's a unique gmail feature.
I always wonder that spammers must surely catch on and work out it would be a good idea to strip the sub-address. You could always blacklist anything not containing one I suppose, but would need to be pretty disciplined about it. The other stumbling block are websites with home-made field verification that rejects emails with subaddresses as invalid.