That key should be self-explanatory, it tags the image that the components were removed.
Reason is to warn users if any issues arise with an update installation that it might be due to removals and to explain in the UI that it's best to integrate the update before removals.
If you are bothered by it, let me know and I might add an option or simply skip that entry as now it's much more compatible after removals.
Btw let me add that I never saw any tool documenting registry entries it adds to save its settings.
There is nothing else added, nor documented.
Of course selected tweaks and removed components with the necessary settings for some of them are understood and expected.
You can also compare complete registry before and after and not take my word for it.
On the subject of documenting, let me prepare you that for 1903, there will have to be added registry tags within components, something like "NCF" with the numerical value, within the COMPONENTS hive's individual removed components entries.
It's quite technical and non-descriptive, so it might not bother you, but there you go, will document that when done.
Reason is that cumulative updates in 1903 return removed pieces that are being updated, so there will need to be a pass to clean that up, unless it's figured out how to prevent it.
There is also the
Host Refresh method of updating, not using Windows Update at all, safest and cleanest, but requires manual interaction for now.