This mutator only works for checkpoint hardcore mode (because of how mutators are set up). Thank you to the kind folk on the Insurgency Sandstorm Modding discord channel for their help in making this run (see below).
I believe you need to add "-mutators=NoClassLimits" to a server command line to make the mutator run (or else add it to an existing list of mutators). You also need to update your mod IDs (usually in mods.txt) with the ID of this mod (110949). You also need to add a parameter of the form '-ModDownloadTravelTo=Oilfield?Scenario_Refinery_Checkpoint_Security', otherwise you will need to restart the level to get the mutator to start operating.
I would like to thank this obscure bug for delaying the success of this mutator: Steamcommunity.com
Please use this mod responsibly.
**Known problem: affects movement speeds and possibly other things as well as the class limits
**Solution: I'd fix this if I can but all I can do is replace the whole squad format file (which includes not just class limits but also movement speeds and so on). So far as I can tell, the devs don't provide the hooks to allow you to (just) set class limits in the blueprint (which is what I'd do if I could). I can provide a separate mutator, or possibly allow you to configure this one, to get different movement speeds, but that hardly seems optimal.
If you want no class limits on normal checkpoint (or any other game mode), I'll have to make a separate mutator for it - just drop me a line and I'll put it together. Sorry - I'd do something more flexible if I could, but so far as I can tell, the devs haven't given us the hooks I need...