The limitation is from when the charges are raised. So that's six years before February 2023, so they can charge us for any breaches after February 2017. This doesn't change depending on when the case is heard.
They have charged us much further back than 2017 though, which means the PL thinks it isn't subject to limitation. This will be because the PL rules do not mention limitation, whereas UEFA's own rules impose a five year limitation period on themselves.
The PL rules are made under English law though, so City will argue that a six year limitation should apply under the terms of The Limitation Act 1980. If the commission accepts this argument, all charges before 2017 will need to be dropped, and we will forever be accused of getting off on a technicality.