Here's how you can make this work in Team Fortress 2. You'll need 3 entities, a trigger_multiple brush and 2 filter_damage_type entities.
1. Make two filter_damage_type's and name one "God_Mode" and the other "Mortal_Mode"
2. For the God_Mode filter, make the damage type "EnergyBeam" and set filter-mode to "Allow"
3. For the Mortal_Mode filter, make the damage type "EnergyBeam" and set the filter-mode to "Disallow"
4. Make a trigger_multiple in the area that you want players to have the god mode
5. Set outputs on the trigger_multiple as follows:
--- OnStartTouch > !activator > SetDamageFilter > God_Mode
--- OnEndTouch > !activator > SetDamageFilter > Mortal_Mode
This means whenever a player (the !activator) enters this trigger, they can ONLY be damaged by "EnergyBeam" which is an unused damage type. As soon as they leave it, they have the mortal filter applied to them which means anything "EnergyBeam" CANNOT damage them now. Either way its completely irrelevant because energybeam is not an actual type of damage. Players who enter the trigger will not be able to be backstabbed, damaged or hurt in any way. It resembles the effects of "God Mode" in sourcemod.
I highly recommend that you put an additional trigger_multiple in the spawn of your map with a "OnStartTouch > !activator > SetDamageFilter > Mortal_Mode" output in it. Sometimes the initial activating trigger can be somewhat buggy and MIGHT not remove the God_Mode filter from the player if they exit it in strange ways (teleport out, lag etc..) and you wouldnt want a random player running around your server invincible and killing people. So the extra spawn trigger will act as a backup and remove any filters that might be left on the player when they respawn/enter the spawn room.
Hope this helps. Ive had this method working in my maps for years, let me know if you have any more questions