npx: deprecate plugin

Fixes #9366

plugins/npx/README.md

 # NPX Plugin
-> npx(1) -- execute npm package binaries. ([more info](https://github.com/zkat/npx))
 
-This plugin automatically registers npx command-not-found handler if `npx` exists in your `$PATH`.
+> npx(1) -- execute npm package binaries. ([more info](https://github.com/npm/npx))
 
-## Setup
+This plugin automatically registers npx command-not-found handler if `npx` exists in your `$PATH`.
 
-- Add plugin to `~/.zshrc`
+To use it, add `npx` to the plugins array in your zshrc file:
 
-```bash
+```zsh
 plugins=(.... npx)
 ```
 
-- Globally install npx binary (npx will be auto installed with recent versions of Node.js)
-```bash
-sudo npm install -g npx
-```
-
 ## Note
 
 The shell auto-fallback doesn't auto-install plain packages. In order to get it to install something, you need to add `@`:
@@ -29,3 +23,17 @@ Started
 
 It does it this way so folks using the fallback don't accidentally try to install regular typoes.
 
+## Deprecation
+
+Since npm v7, `npx` has been moved to `npm exec`. With the move, [the `--shell-auto-fallback` argument
+for `npx` has been removed](https://github.com/npm/cli/blob/v7.0.0/docs/content/cli-commands/npm-exec.md#compatibility-with-older-npx-versions):
+
+> Shell fallback functionality is removed, as it is not advisable.
+
+When using npm v7, you'll get this error:
+
+> npx: the --shell-auto-fallback argument has been removed
+
+If you get this error, just disable the plugin by removing it from the plugins array in your zshrc file.
+This plugin will no longer be maintained and will be removed in the future, when the older `npx` versions
+are no longer available.