JPさんSNIが必要なのはたしかっぽい
SNIを送らない場合
$ :| openssl s_client -connect mstdn.jp:443 > /dev/null
140454093619648:error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:../ssl/record/rec_layer_s3.c:1399:SSL alert number 40
SNIを送る場合
$ :| openssl s_client -connect mstdn.jp:443 -servername mstdn.jp > /dev/null
depth=2 C = IE, O = Baltimore, OU = CyberTrust, CN = Baltimore CyberTrust Root
verify return:1
depth=1 C = US, ST = CA, L = San Francisco, O = "CloudFlare, Inc.", CN = CloudFlare Inc ECC CA-2
verify return:1
depth=0 C = US, ST = CA, L = San Francisco, O = "CloudFlare, Inc.", CN = sni.cloudflaressl.com
verify return:1
DONE