This was suggested on the original PR here:
#11466 (comment)

Both @laanwj and I agreed that walletdir is easier to remember and type :)

I strongly prefer the singular name. I'm going to forget the 's' every time. I think it's fairly clear that it's a directory that it can contain multiple things, but in any case that belongs in the documentation, I don't see a reason to rename it.

Agree with @laanwj.

I did mention three reasons above (hopefully they make sense), and I don't exactly know when you would be typing this option, but if getting rid of the s here is important do you also want to make the default location <datadir>/wallet?

Ok, I don't have a strong preference. Maybe you had a bad experience with the letter s. Was mostly concerned about confusion between -walletdir and -wallet options if different wallets stopped being stored together in a single bdb environment. Will close.

Was mostly concerned about confusion between -walletdir and -wallet options if different wallets stopped being stored together in a single bdb environment. Will close.

Yes, that's a valid concern.

Fwiw, it's "computer store", not "computers store", "grocery bag", not "groceries bag", "cookie jar", not "cookies jar". Though I'm sure there's plenty of counter examples.

Another plural exception:

  QDesktopServices::DocumentsLocation
  QDesktopServices::FontsLocation
  QDesktopServices::ApplicationsLocation
* QDesktopServices::MusicLocation
  QDesktopServices::MoviesLocation
  QDesktopServices::PicturesLocation

so wallet is like music 🎶 ...

(not english native speaker, so maybe there is a reason to be singular in this case)