I am writing code that uses Underscore JS in Drupal 9 that should be loaded from /core/assets/vendor/underscore.
I am using Chrome DevTools to see the sources and though jquery, jquery-once, and modernizer are all loading from /core/assets/vendor/ the underscore library is not.
I am almost positive that this was working last week. I have updated my code since then but it is the same use of underscore. I do this in Drupal 8 and it works for sure.
I have cleared the cache multiple times. I have verified that the files for underscore are in my filesystem.