After using the codemod to generate eslint.config.mjs, I ran into the following error when running the linter:

Key "globals": Global "AudioWorkletGlobalScope " has leading or trailing whitespace

This error comes from the globals package itself. We rely on a version that literally contains the said typo. The latest version (15.8.0) has been fixed. Using yarn resolutions to force the usage of a fixed version solves the linter issue.

The next problem is:

TypeError: context.getAncestors is not a function
Occurred while linting /Users/marine/Desktop/Repos/opensource/dazzlingfugu/ember-reading-time/.stylelintrc.js
Rule: "ember/require-return-from-computed"
  at onCodePathStart (/Users/marine/Desktop/Repos/opensource/dazzlingfugu/ember-reading-time/node_modules/eslint-plugin-ember/lib/rules/require-return-from-computed.js:70:16)

Unfortunately, this issue is currently blocked by eslint-plugin-ember not supporting every aspect of ESLint 9. The specific problem above should be fixed with eslint-plugin-ember#2153 (will close eslint-plugin-ember#2136) but it hasn't been released yet.

Just a note that my fix wasn't complete, but it does open up using a workaround to getting the ember plugin running in v9.
ember-cli/eslint-plugin-ember#2136 (comment) if they decide to release at some point