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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.
Additional context: read-only package repos (as archived on 2019-12-09) moved to official site.[2]
[0] https://github.com/termux/termux-packages/issues/4467
If the latter, I'd guess that some of them still want to support Android 5/6 phones. Some of them still exist and work fine.