Just because bitcoin core doesn't have a certain feature it does NOT mean there is no standard for that feature. For example core doesn't support any form of mnemonic including BIP39 but that does NOT mean there is no standard for mnemonics!
Great point and excellent example.
I've already posted 2 "standards" for it above but since bitcoin core doesn't implement those people think there is no "standard".
I wasn't aware there were already BIPs covering segwit message signatures. I suppose it would have been more accurate to say "there isn't a specific, established standard." The fact that two BIPs exist might be the reason for delayed implementation. I suspect you're right about most developers wanting to just copy and paste code from Core, that would make sense. I can also understand the hesitancy to implement one, only to have Bitcoin Core implement the other at some later time, effectively making that the "standard."