If we're talking specifically about executable scripts, here is #bash's (libera.chat) factoid on the matter:
Don't use extensions for your scripts. Scripts define new commands that you can run, and commands are generally not given extensions. Do you run ls.elf? Also: bash scripts are not sh scripts (so don't use .sh) and the extension will only cause dependencies headaches if the script gets rewritten in another language. See http://www.talisman.org/~erlkonig/documents/commandname-extensions-considered-harmful
It's for these reasons that I keep my executable scripts named without extensions (e.g. install).
I sometimes have non-executable scripts: they're chmod -x, they don't have a shebang, and they're explicitly made for source-ing (e.g. library functions). For these, I give them an extension depending on what shell I wrote them for (and thus, what shell you need to use to source them), e.g. library.bash or library.zsh.
Yeah. It also seems to me like budget Murktide Regent for Standard, because it's easier to cast (you don't need to exile) but doesn't grow