426 Upgrade Required
I only answer in another protocol — and the Upgrade header says which.
The server refuses to serve on that protocol version and demands the switch on the same connection: RFC 9110 requires the response to carry the Upgrade header saying where to climb. It is the 101 in a bad mood — the same door, used as an ultimatum.
It was imagined for migrating connections to TLS on the same port, in RFC 2817 — a plan that https:// on its own port made unnecessary. Today almost nothing returns it: whatever demands a newer protocol usually refuses the connection before HTTP even starts.
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Type: text/plain