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.
Also, there's always https://mailhero.io/
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.
I find common culprits are service companies; gas, electricity, real estate websites, insurance companies etc.
EDIT: Oh, and warranties! Man warranty websites tend to suck. Samsung for instance won't let you register your product to an email with a +.
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.)