substr() is deprecated and is not part of the core JS since ~2018.
No wonder why no one noticed this :)
Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr

So, I've refactored whole codebase to not use substr() and use slice() instead here:
PR #TBA #37136

Originally posted by @Pranav-yadav in #35993 (comment)