Speaking as someone who uses + addresses to filter stuff from mostly well-known websites, I have never seen this. I have seen this a few times on old, crusty, finance websites etc. but I hardly ever need to use a + address with them anyway. (It does make me wonder about how good their internal security is, though.)
BestBuy allowed me to sign up with a trailing "+bestbuy@gmail.com", but their unsubscribe interface rejects it as invalid, so I can't unsubscribe from their promos. I just filter/mark as spam and move on.
I've seen this several times as well. If it's a referral link to unsubscribe and is meant to include your email, double check it's okay. If your plus is missing insert "%2B" where it should be.
It's less common, but some websites ask you to type your email address once they realise they can't find you on record. If the "+" doesn't work in that field (server side rejected), then you can try %2B there as well.
If they refuse it, I think it's generally because they _know_ you're filtering them, and don't want you to sign up with a single-use address. In which case I don't bother signing up because I know they'll be spamming.
As of a year or two ago, Garmin wouldn't accept email addresses with "+" signs in them when registering for some (but not all) of their services. I think they would let you register for the main Garmin Connect service, but not for their support forum or something. Leading to a nice situation where you couldn't get technical support via the forum for the same account you had created on Connect.
Interestingly, though, the support email address they had hanging around at the bottom of the login page actually routed to an engineer (somewhere), which was nice, although they did claim repeatedly that the "+" character wasn't valid in an email address, which is of course not true. I was floored just to get a response from my complaint, though, even though it didn't seem likely to lead to them fixing the problem.
Speaking as someone who uses + addresses to filter stuff from mostly well-known websites, I have never seen this. I have seen this a few times on old, crusty, finance websites etc. but I hardly ever need to use a + address with them anyway. (It does make me wonder about how good their internal security is, though.)