It’s also possible that it is entirely accurate for Germany (meaning that Germany mappers really do map this way). And it may well be accurate for other countries too - I didn’t do a systematic check on how those 3500 place=municipality notes are distributed globally. Note that even in Austria these nodes are used in some limited circumstances, so a bare “do not use” would not be correct.
(Also, did you mean “do not use” on nodes specifically? I don’t think there is any issue with place=municipality on boundaries).