[chii]

Except sites that mistakenly disallowed the + symbol.

Also, it's very easy to mechanically identify all such users because of the + symbol, which, if you are trying to prevent your real email address from revealed means it's not that useful...

[blacksmith_tb]

Presumably they don't block [.] however, and since gmail ignores it, you can always just use t.estexample@gmail, then te.stexample@gmail, etc.

[Fnoord]

Regarding the former, my ISP allows me to set forwarding email address. I could temporarily use these until X date or until they receive (a lot of) spam. Though all spam gets filtered anyway.

Regarding the latter, when they email you directly without the + you can be very strict. You could even apply whitelisting.