the issue with 1.21 is for some reason it requires identical Custom Steve folders in both the mods folder AND the main .minecraft folder at the same time with identical content. it's really weird, also I am running 1.3.0 and it's working fine for me, I even managed to add an InuYasha model to it using sakuya's animations (which look just plain odd).
Try putting the CustomSteve folder and .jar into both the mods folder and .minecraft folder for 1.21, it did not work.
What forge version are you running? There might be something with that. I have 1.7.10 - 10.13.4.1614.