When I first wrote the Lua module for libtls [1], I had issues with using TLS_API to version the code [2], so I ended up using using LIBRESSL_VERSION_NUMBER instead [3] and mandating at least LibreSSL [4] 2.3.0.
But now that I'm switching away from LibreSSL [5], I can't rely upon LIBRESSL_VERSION_NUMBER anymore. After some thought, I have decided to no longer support versions of libtls prior to 20180210 (or LibreSSL 2.7.0). The non-LibreSSL libtls versions I've found all seem to be newer than 20180210, so that shouldn't be much of a prooblem.