You might have seen screenshots of the "EA Sports Football Club Catalogue" where classic kits cost 40–60 credits. That service was shut down in 2016. There is to open that catalogue menu because it required a live server connection.
There it was. Not locked. Not greyed out.
Now the classic kit will appear in kit selection for that team.
If you want, I can:
Use the mod’s configuration files to assign the kit to a specific team ID.