FLK doesn't depend on any MC code, making it technically compatible with any version of the game.
Marking at as compatible with versions earlier than 1.14 would allow Legacy Fabric, OrnitheMC and Babric modders to declare it as a dependency on Modrinth and Curseforge.

Regarding log4j usage, both OrnitheMC (indev-1.14) and Babric (b1.7.3) include it in their runtime environment.
Legacy Fabric (1.3-1.13) doesn't so it would be missing for 1.3-1.5.