Dominik
Yes, but interestingly enough the UI did not give me a TXT record for verification. Saw "DNS verified" straight away and it committed the CNAME file into the gh-pages branch....
Just realized that adding domain to GH pages and verifying are two separate processes. Now also verified for sabre-io org.
Looks like this is an oversight and the doc page (for server side managed subscriptions) was never created. There is https://sabre.io/dav/ics-export-plugin/, but that's about providing iCalendar feeds, not about server-side...