Earliest version of termux on f-droid in the internet archive requires android 5.0 or later. Current versions require andoid 7.0 or later. How does the user with old android phone, e.g., 4.x, use termux. Current versions of primitive ftpd will work with android 4.0.3 or later.
if your old phone has no unofficial support for more or less modern on version, either forget about it or go all in native linux route postmarketos etc
