Well, AvxSynth exists. And now so does VapourSynth. Two different approaches, with differing levels of compatibility with AviSynth. Disregarding, of course, that the plugins (which are really what makes the tool so versatile) are independent of the core even with the original and would also have to be ported to be useful (although there was brief talk about how it would be cool to have a thunking layer for the plugin .dlls to work with AvxSynth since relatively few of them rely on the Windows subsystem).
I think much of the relative lack of activity in getting the external filters ported is sort of a case of waiting with baited breath that one of the solutions actually succeeds and takes hold. Or the developers being Windows-centric, or with some plugins, being just plain old and active development on them having stopped years ago because there just wasn't anything else to add.
Speaking of AvxSynth, though, there is some activity right now regarding changes to FFmpeg's AviSynth parser that would allow it to accept AvxSynth input on Linux. Hopefully this happens soon and then there'll be more traction to work with on that front.
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