Current Debian stable, from July 2019, has 3.13. Debian testing,
eventually becoming stable, has a 3.16 version in it.
Therefore, I can easily use 3.16 myself.
I know you're asking about/our/ /development/ machines, but for
end-users, I'd say most Ubuntu users will still be on 18.04 LTS, some
might be on 16.04 LTS (given 5yr support didn't run out yet). It would
probably be nice to wait at least a bit before nuking support for
18.04 and its derivatives. And I suppose CentOS that Richard mentioned
is also important to keep in mind?
If you want to move to it sooner rather than later, how about an
alternative deprecated CMakeLists.txt for older systems, one which
gets phased out in ~1.5-2y or so?