But when it comes to non-legacy addresses there is no standard signature format so electrum generated signatures can't be verified by other wallet software.
I have heared this many times, but I don't agree with it.
In my opinion, it doesn't really matter whether your address is legacy or segwit when signing a message. You sign the message with your private key. Your private key can generate legacy and segwit addresses and your message can be verified using each of them.