How to play outdated mods

Subscribe to a mod!
Go to your Steam installation folder and then to "SteamLibrary\steamapps\workshop\content\976730\" The last numbers may be different, you just need to locate where your MCC mods are.
Find the mod you want. There will be a list of all MCC mods you have, you can either go one by one or use the search.
Update the mod!
Open up ModInfo.json in Notepad. What you need to find is this part:

Change the "Major" to something like 9. Once done it should look like this:
Save the file and enjoy the mod!

Keep in mind that some mods may have been broken after some MCC patch. All Halo CE mods should work fine but I'm not sure about the later games.