It was a big scandal a while ago when a Spanish newspaper leaked Lionel Messi’s Barcelona FC salary, which was reported to exceed any known sports-club salary ever paid out to an athlete, but is it customary for European teams to keep their salaries secret (as opposed to, say, American leagues like NBA, NFL or MLB, in which all player salaries are totally public?)
I don’t think so, at least not for soccer players, as you can find salaries for all of them from many sources. Perhaps Messi was getting additional unreported salary; e.g. it was being reported as marketing, overhead, etc.?
With FFP rules, I assume clubs have to report salaries, but I’m not sure if that is made public.
That, to me, has always been the greater mystery. I guess the players egos are behind it.