>>26704874It's been gradual throughout history. I mean, when the game started, having more than 40 HP on a Basic was considered above-curve and swinging for 40 off 3 energies was the standard. If you've played until Gen III, you should know that that stopped being the case very quickly.
But there was a bigger than average jump around the end of Gen IV with HGSS. Pic related is the picture everyone shows off, but it's not entirely fair. Despite that, it shows an example of the difference Gen V brought along. The end of Gen IV and the start of V also gave us Oak's New Theory and Juniper, the first reprintings of the original Base Set Oak, which radically changed the trainer metagame.
The real instance of creep, though, was with EX Pokemon. They're like the Gen III EXs, but they're all Basics with 50 more HP than the Stage 2s of the time and even more damage. The mechanic could have been balanced without introducing creep, but the implementation was poor. We're only just now recovering. As of RIGHT NOW, there hasn't been any creep at all over the past 2-4 years.
All of this only really deals with the printed numbers on cards. Things do more damage and have more HP, but the game's been mechanically simplified and trainers have regressed significantly (other than Juniper/Sycamore). If there were a serious format that allowed you to use any card ever printed, a majority of decks would feature pre-Gen V trainers and support Pokemon. Base Set trainers were easily the strongest ever printed.
Don't forget that Pokemon's been using a Rotation system since Gen I, making terms like "power creep" completely irrelevant. It doesn't matter how weak older cards are if you literally can't use them anyways. 