In theory it could be possible to run it, but it's not something we'd support.

Whether all the 3rd party libraries still support it is unknown. Even if they do compile, they may no longer have optimisations.

It's likely a lot of work and there is functionality that won't work or won't work well due to lack of system resources on systems that old. I.e Anything other than the older codecs is probably a non-starter.

@sr55 Hopefully no special support will be required. GTK software which specifically targets macOS often has issues (due to usage of quartz, which uses Cocoa, which is broken), but “normal” (i.e. X11-based) should just work.

Whether all the 3rd party libraries still support it, or even if they do compile

Looking at what FreeBSD has as dependencies, everything builds in lastest versions and presumably work. (Well, I did not run test-suites, but my video playback software on powerpc uses the latest ffmpeg etc.)
My only concern, without looking at the code yet, is CoreAudio, which may or may not work as is (and be fixable if it does not). Specifically, if the current implementation relies on blocks extension, it won’t work until GCC upstream implements it. Otherwise it should work too.