I still wouldn’t count on that though. No good reason to hold off version upgrades IMO.

languageServerOverridePath may no longer be useful. Since v0.14 the extension uses Mix.install which builds and installs elixirLS using the correct elixir and OTP combo

That’s workable. Just delete all build artifacts, stop VSCode (if you are using that then just stop your editor, whichever it is) and start over:

rm -rf .vscode .elixir_ls _build deps
mix do deps.get, compile

Then start your editor again.

for question 2 problem:

Vscode elixir introduced setting languageServerOverridePath. You could find it in vscode settings ElixirLS.
This setting can be configured per workspace, so each project will have its own elixirLS release, which is handy if you’re using asdf for elixir installation and have different elixir versions per project.

Basically, you need to download ElixirLS precompiled release for desired elixir version from here
Releases · elixir-lsp/elixir-ls · GitHub and upzip it to some directory. Then use absolute path to that directory to set languageServerOverridePath setting. Then remove .elixir_ls and _build directories in your project and restart VsCode.
Every time, when vscode elixir ls extension upgrades ElixirLS version, you’ll need to download a new release and repeat all the above.

IMO, it’s better than compiling ElixirLS manually and replacing it in vscode extensions.

Elixir 1.11 is still listed as supported here: https://github.com/elixir-lang/elixir/blob/main/lib/elixir/pages/references/compatibility-and-deprecations.md

And you can use it with Erlang 21 to 23.