There are a fair few devices out there still running old versions of Android¹ – particularly a number of ebook devices that are commonly rooted to do more/other than run the officially included software which may be inadequate², deprecated³, or simply dead⁴.
Yup. Notably, these devices are pre-Treble, so there's no prospect for any sort of GSI support (which would open up things like Droidian). So Termux is pretty much the best you can do if you care about providing a sensible environment in a device-independent way.
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[1] >3% of android running 6 or 5, according to several sources found in a quick search, of which https://gs.statcounter.com/os-version-market-share/android seems to be the most up-to-date.
[2] Not supporting newer formats/features perhaps, or just being a very tightly walled garden of minimal features.
[3] So not supported at all any more.
[4] If they rely on external services that have since closed, for instance.