I started on this bit of inspection due to getting really odd behavior when loading a large number of community tracks and mods.
When the game runs out of freeslots it does NOT appropriately error. What happens is you end up with things broken/missing in any mods you've loaded after running out of slots. I can't think of any good reason where you'd want this behavior. So here I've set the game to I_Error when the limits are exceeded, with a message that suggests loading fewer addons.
This also increases the Freeslot define to 1024. Now that the game uses LAA and can address more memory, I don't see much issue with increasing the freeslot limit.
I've tested the error messages with a simple lua script that loop allocates, as well as a real mod loadout from my server. I've also tested the limit increase with the real loadout that had 560 or so Mobjtypes. Mobjs past the existing limit worked fine in this testing, so the limit increase looks simple and effective.
