The plural of "moose" is also "moose" but it's not because of English. Moose derives from Algonquian, a Native American language. It kept the same plural ending it had in its original language instead of adopting the normal "s" ending of most English plurals.
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Or they could just ignore it because the point of language rules is to communicate unambiguously and the meaning of "deers" is pretty clear while "deer" is ambiguous.
Common knowledge doesn't mean people use it. It's easy to forget even if you studied about it in school.
For example you is singular and plural. But we rarely use you for multiple people nowadays, we just go "you guys", "you all", "all of you", or something else to disambiguate.
Languages move towards easy communication and simplicity.