Or worse, they'll accept it, but then some backend system trips over it, and now the product you ordered never ships to you, but customer service doesn't know how to refund it either.

I've been "fighting" many websites in the last 20+ years, which use(d) javascript libaries which accept "only a-z, 0-9 and _" as valid characters in a local part. Some even changed their code after I complained and pointed them to the relevant parts of RFC822 (and all successors)

IMNSHO: sysadmins who do not know that the local part of an email address is not of their concern (as long as it complies to RFC 822++), are not worth their money. And web designers? Don't get me started on that topic ;-0

Edit: ok, they even allow "." and "-" in local parts.