Footer: add 5mm Y-offset to place within margin
OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 184.108.40.20694, revision: 5cc4d6b
If you tick "Show page margins" in a score, notice that footer text is now above the bottom page margin: it should lie underneath.
Footer in 3.x:
Same footer in 2.3:
Perhaps due to this fix, #284635: Last system of page is one staff height too far from margin?