Good point, cmake 3.13 would mean drop support for 18.04, cmake 3.10 would mean drop support for 16.04. Both are currently packaged on deb.kamailio.org.
We should probably formalize some support timeline (e.g. on a web page) also for distributions. Xenofon will list again the issues we would have going back to 3.10, right now the requirement is 3.13 as I understand it.