As you have discovered, older versions of macOS are not supported. However, @Tiberiu_Vicol did manage to compile some versions last year. I managed to compile 5.99 for High Sierra but it had some issues (layers palette was essentially unusable)
My final and best solution was to use the Catalina patcher which allowed me to run Catalina a on otherwise unsupported hardware. YMMV and I can understand a reluctance to do this for one application but I have found it rock solid and am running KiCad 6.0.2 on Catalina on a 2011 machine.
When I bought this machine, it was the Top Of The LIne (max everything). Spent a lot of time with software so that all my engineering software, Harley Fuel Mapping software and much, much more, ran well.
Then, I upgrade High Sierra to Catalina. OMG!!! what misery that was - broke many things so, after hours of Restoring Sierra and getting back to working condition, I decided 'I will Never Again upgrade my three Mac’s. Period.
The good news is, v5.1.12 works perfectly well for my needs. The (perhaps) even better news is that, not being able to upgrade Kicad, puts my Kicad Forum participation at a ‘Time To Move-On’ to other interests level. Can’t help folks with newer Kicad so, why bother.
I have no problem compiling the code but, why bother if I can live with what I have…