The 600 energy "limit" was mentioned by a dataminer a few years ago. What happens is that when you cross 600 energy and haven't spent real money on the game, your account gets flagged. This doesn't necessarily mean that you are cheating, but it does mean that in group events you will get sorted with other flagged players.
On my throwaway account I have passed 600 energy quite a bit, as the game seems to just throw energy at you these days. The most energy I've had on that account is 2000ish. It's almost impossible to avoid the 600 energy limit now, but so long as you avoid group events it won't really matter.
It's still good practice to try and keep your energy amounts reasonable. I've seen people post screenshots of them with like 100000 energy, and then a few days later posting about how they were banned. It's possible that they logged on in a full marks or house pride without realizing it, but I don't think that having unrealistically high numbers is safe, and with the energy hack with this apk there's really no reason to stockpile that high.
The best advice that I can give is that if you do pass 600 energy accidentally, just use it in a lesson or gathering food and try to get back below 600 as soon as possible. That way you are not going to accumulate too much in the long run.