Some people set filterkeyword to name-of-business, to know who sold them out if they start receiving spam on that address.

I use the + technique, but I've been doubtful of its effectiveness. It's easy enough to remove the text after the +.

Yeah, I’m sure many spammers “fixed” this a long time ago. The logical next step would be to bounce all emails which go to the plain version of an email address, but I guess also anything after + which one hasn’t “whitelisted”.