the mcmeta file is what makes the pack red when you try to load it into the game, it doesn't actually do anything to the contents of the pack itself. chances are, something that the pack modifies has changed between 1.19 and 1.21, whether that's simple syntax, block ids, or something a lot deeper in the actual code functions. you always have the option of playing minecraft 1.19.1, which would be the easiest fix, or you could look through the files in the pack to see if you can figure out what isn't working anymore by comparing it to game files from 1.21.4
best of luck
