NPM Install add custom warning message

There's not necessarily a way you can do that, or not with given fields like "deprecated"

What you can do, which is a little bit of a workaround, is adding a post-install script, that outputs a string to the console if you mark a version as alpha.

// package.json
{
  "version": "1.2.3-alpha.2",
  "scripts": {
    "postinstall": "node postinstall.js"
  }
}

// postinstall.js
const package = require('./package.json')

if (package.version.includes('alpha')) {
  console.log('You are using an alpha version. Beware!')
}