For the record, I am pretty sure Caddy will be unaffected by this. Any programs using xenolf/lego as their ACME client should be fine as well, as long as one other validation method is still available. (lego also uniquely supports a wide variety of DNS providers for automated negotiation of the DNS challenge. Caddy supports them too, as long as they are plugged in and configured.)

I've been asked if we'll turn off TLS-SNI in Caddy and the answer is no; as their announcement says, some accounts will still be able to use TLS-SNI for a limited timeframe until it is turned off completely. Caddy won't try the TLS-SNI challenge as long as the ACME server doesn't advertise it in an exchange.