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No diplomatic nagging

The AI will stop offering you non-aggression pacts, defensive alliances, military alliances and joining their wars. They still offer more useful stuff like trade agreements and military access. You can still offer them everything and they will be just as likely to agree.
The script also works if enabled later in a campaign. But it only runs once, so the effects are restricted to the factions present when the script was loaded the first time. And they remain in place when you disable this mod, because the game writes them into the savegame. (They could easily be disabled with a similar mod.)

There is also a version in my workshop which disables all treaty types.

The code would have worked with vanilla, but I had to put it in a DeI script file to get it to load. If it is placed in its own file it would need to be launched from startpos, and then it won't be compatible with other mods that launch scripts like AVR. This also means it only works in the grand campaign.
I might not get around to making other versions, so if anyone wants to put this code into a vanilla script and make it load, feel welcome to 'steal' the code and upload a vanilla version.
Credit mostly goes to Jake Armitage, who originally developed the code this is based on.

Compatibility should be fine as long as this is loaded on top. It disables gc_first_turn_setup.lua in mods below it.