I have read their research. It's 651 energy to be exact. But, it has to be shimmered before the flag is triggered. If you reach 650+ but quickly spend it all, you won't get flagged. I don't know how to see the game's code, so I did my own experiment back then.
I know my main is flagged so I made another vanilla account to test a theory. I played without mod/GG intervention whatsoever until I passed Y1C6 so that I can participate in the next MP event. I joined a group where you can be grouped with some legit players during MP event so that we can share the 1st place. There's an upcoming FM and I was grouped with non-flagged players. After that FM ended, I ascended to Y2 and the first thing I did was switched to mod apk with that non-flagged account and buy 1 set of clothes in a Brilliant Bargains. I didn't do anything pass that. I wanted to test if my account would immediately get flagged if I used the mod service. Well, it certainly did turn my once non-flagged status to a flagged one. I've told my story of my main and my alt got sorted together in a FM event back in 2022. Yes, it was for the sake of confirming that status. Hence, I came to the conclusion, as long as we've used mod service, your account would undoubtedly be flagged.
There's also an interesting research they did. This player was never once stacking over 650 energy and bought stuff from the in-game store with muggle money. Their account was non-flagged btw. After they bought something, they began to stack over energy until they reach the threshold and during FM, they had over 650+ energy and kept it that way until the FM ended (they were still sorted with another non-flagged players by this time). The upcoming FM however, they were grouped with flagged players surprisingly. But, it's not like they were automatically grouped with cheaters, only grouped with some known flagged players. In the end, they didn't know when they were flagged, was it during the last FM or before the MP event commence. So, it seems even buying stuff in-game doesn't give you immunity to being flagged? Although, some claims it gave them protection, but this whole flagged system is weird in the first place. Even JC denies this system is implemented into their game. Well, not that I care tho. As long as we don't participate directly, we should probably be safe I guess.