From what I understood so far, the GI allows the Circs to leave FBB, most of them just are not motivated to do so. So I wonder if there was a political shift in the GI after they tried to get rid of the decompers 300 years ago.

It's that a lot of them want to leave but only a handful are actually allowed to, and the Circs have no idea what the criteria for emigration are. There is some sort of screening process they have to go for, and to make a guess from this chapter, the GI might be basing it on their standardized decomping test scores. If FBB is the GI's internment camp for decompers, then someone who can't decomp doesn't need to be there. There's a good chance Diode could've simply walked away from FBB if she'd managed to get to the emigration boat during the Bow Awow.