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.