I do it. I think TMIT does it as well, or at least his post introduced the idea to me a few years ago. I don't think most deity players do.
Also you don't cancel the GPT after 10 turns. You trade your max GPT + (solo) resource for other resource. Then you pillage your copy of the resource, so you don't even have to make a big investment up front.
I don't consider it cheating either. All GPT for resource trades are cheating in the sense that the AI is almost always overpaying, because it has no way of evaluating how much a new resource is worth. Health = the same as happy = the same as double happy or double health = the same whether it's below its happy cap or 20 above it. The subsidy mechanism works only because the "available GPT" is finicky, pseudo-random BS. It seems silly waiting for the AI to randomly have a decent amount of GPT to trade. Optimizing involves staring at the resource tab literally every turn. There's far more game changing "abusive" things that we do every game that we don't think twice about because we've been doing them for years. Subsidizing utilizes the tools the game makes available to you. It's not so much a bug/exploit as it is being really sneaky. The AI already gives bad GPT for resource trades. The pillage button exists in the game. Surely the developers must have wanted us to do this
Some people have argued that the fist icon in BUG ruins the game because it makes DOWs predictable. Of course it does. But again "WHEOOHRN" is in the game. For some reason. I have to assume the game designers considered the possibility we'd figure out what that means.... and having to check it every turn is burdensome, so we get fists in the bottom right corner instead.
It's also important to note out that it is much more useful on some game modes than others -- and I think it helps balance them, which is why I don't see anything wrong with it. You need a lot of resources, and the AI needs to have a lot of cities. On pangaea (what I usually play) this trick is pretty useless, as you don't have that many resources, and the AI doesn't have a lot of cities. But imagine a larger continents game, where you fight a brutal war against two psychos for the early and mid part of the game. You manage to get to a lot of cities, but you're hopelessly behind. I think it's perfectly fair to trade your resources for a lot of GPT to catch up -- and it's perfectly realistic.
It can also be used to push a runaway AI into financial trouble. Again when you get discover another continent you might see four civs evenly balanced, or one civ 3x the rest running away with the game. With subsidies you can extract say 200 GPT from that AI, which significantly slows down even a deity AI. This may seem very lame, but from the perspective of balance I like it. After all you had no input as to what the AI land ratios would be like on the other continent. If they're all equal you will have an easier game, and coincidentally subsidies won't be as important, but against a runaway now you have a tool to bring him more in line.