“Master“ implies that the contents is authoritative somehow, as in “master copy” (meaning 13 in https://en.wiktionary.org/wiki/master#Noun). “Main” doesn’t have that connotation.
When one is willing to discard that connotation, then, if anything, “default” would be a more accurate name, because the fact that it is selected by default in certain situations is really the only technical difference compared to other branches.
When one is willing to discard that connotation, then, if anything, “default” would be a more accurate name, because the fact that it is selected by default in certain situations is really the only technical difference compared to other branches.