We revisit a lot of things inevitably, even unintentionally, as there’s lots of us working on this / who have worked on this previously. Not to mention re use of episodes and references and holiday things are bound to happen.
We do not however have a permanent long term list. In a way that would have been neat to have from the start. Buuut we don’t. And if we did start it’d be a bit of an unnecessary chore of sorts.