What is ~/.local folder in Mac OS used for?

It contains shared resources, not files which can be shared with other users. [I'm not quite sure exactly what they're shared with, but Wine uses the same structure]

You can confirm this just using Finder set to show invisibles Cmd ⌘ Shift ⇧ . & getting Info on the folder.

That's not a path defined by Apple. It was probably chosen by whoever created the nvim installer you used. The name suggests that it's stuff that would go in /usr/local in a traditional Unix filesystem layout, but the nvim installer puts it in ~/.local when you don't have admin privileges to modify the system-wide /usr/local when you install nvim.

This folder is often found on Linux. It is part of the XDG Base Directory Specification. Another related folder is ~/.config.

~/.local/share is for "user-specific data files". The related /usr/share folder is "for all read-only architecture independent data files" in the Filesystem Hierarchy Standard.

See also this Ask Ubuntu question and answer.