Sorry about the trouble @sparker. This was a one-time change that passed through the cracks in our automated testing. Going forward, environment upgrades that aren’t critical security fixes will be entirely optional. We will also add more tests to catch these issues before they make it to production.
We will update our guide for existing Phoenix projects. It still works for new projects because the latest version of phx.new installs a version of node-sass that is compatible with Node 14:
"node-sass": "^4.13.1",
This installs node-sass version 4.14.1 which added support for Node 14 in April 2020.