It is trivial for a third party to download and install Electrum if they don't already have it in order to verify a signed message if the wallet software they are using does not support public key recovery via segwit address. Or alternatively, and slightly more complicated, they could convert the segwit address in to its corresponding legacy address and use that to perform public key recovery and signature verification, as I explain here: https://bitcointalk.org/index.php?topic=5417111.msg61126295#msg61126295