The Hall of Fame allowed exploits (or 'skills' in this case) page says:
"Signing GPT for upfront compensation from the AI, and then using a declaration of war or disconnect of trade route to cancel the deal. Comes with a reputation hit in some cases, in others it is free."
So, such schemes are implied as legal for the HoF. It's the XOTM competitions that disallow them.
I think there is a question about whether such got intended. But, every game does come as intended as winnable or for players to at least understand why they lost. I think skilled enough players could win on some map on Sid without such a tactic. But, every game as winnable perhaps extends to all victory conditions as possible. My opinion lies in that the most difficult victory condition is Sid 20k on a pangea map. Conquering AIs can be largely sufficient for most victory conditions and can happen later rather than sooner. Even with a 100k game, I'm guessing that the culture could in principle get built quickly enough after the conquest phase finishes. But, a 20k game I'm guessing (I think I remember reading some calculations somewhere) just isn't possible on regular buildings, small wonders. Some great wonders are necessary and early enough. I remember reading Sirpleb's multiple victory game suggesting that it felt surprisingly little in terms of great wonders built to have a victory by 2050 AD.
It's likely trickier than the HoF strategies/power starts suggest, I think, because of low food and shield starting positions.
The scheme can get used on those maps still and getting out a small handful of cities is possible on a maximum opponent map, in the worst city possibility case by founding on one's one cultural borders. Territory might be so bad that moving the capital and finding a suitable enough 20k site or conquering enough cities to get up a 400 shield palace prebuild, then prebuild, and finally stall finishing the build before a tech becomes available, or in the worst case a 600 shield prebuild might be necessary for some medieval wonder or two. But that means substantial early medieval war I think. And I would doubt that an 18 shield maximum size 12 city site will do. Some positions are, of course worse, and there's the whole flood plain issue of killing off shield production. Quick population relocation also might require both an MGL to move both the capital site (could probably sit at size 1), and workers ready for joins, but that you had the food to even do that for one city seems questionable for plenty of starts.
I don't know, even with that tactic an average start Sid pangea 20k game seems rather tricky and uncertain enough, that this sort of pillaging tactic being in the game might have been intentional, especially if you need to attack fast like I'm guessing, but if you attack too early, you're dead.
