See https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html
Fulcrum sets a limit to 80 characters but allows any arbitrary string within that limit. Electrum can handle wrong address formats via the usual "Pay to" URI parsing code.
I don't see a particular reason to enforce the 80 chars size limit. Note that the call only happens if the users clicks the "Help > Donate to server" menu, so the attack surface is very limited for Electrum in case of very large strings.