To exclude a specific entry you can right click it on the analyze list and choose exclude key. You may have to "manually edit" it once it's in the exclude list. To do this find the entry on the exclude lsit right click it and manually edit it to match your want
Has Piriform deliberately chosen that some keys shall not be protected from cleaning.
Or is this some obstacle by Microsoft in what can be written to CCleaner's registry settings,
in which case does this also apply to the Portable version that keeps settings within CCleaner.ini ?
I don't know. I've tried to exclude a very small handful of registry keys that CCleaner refuses to comply to the exclusion. For instance one is a MUI Cache entry that constantly shows up in the registry which is written in that cryptic Microsoft way so CCleaner thinks it's invalid:
1. It can't be excluded via the CCleaner GUI.
2. CCleaner completely ignores the exclude if I manually type it into ccleaner.ini.