Based on a conversation on IRC, I’d like to announce that we will drop support for Debian/stretch (9.x) in the next (1.25) release. With that we are in line with dropping Ubuntu/xenial support, which also brings Ruby 2.3 and after thus we can use Ruby 2.5 as minimal version.
As 1.24 is supporting Debian/buster (10.x), upgrades are possible with that release.