It's very confusing to use: You need to know regular expressions and type commands from scratch. This is not an acceptable thing to be so visible. I needed help to set it up. The "immediately on selection" action is broken and janky. The "exclude windows" list is X11 specific and completely broken. It also only relates to URLs which is very unclear from the UI. The examples given in the manual are just about opening links, which might have been a problem years ago, but isn't now.
If we remove the feature, you know we'll find the people who have fought through the bugs and are using it for something. IMO deciding whether to poke that hornet's nest is the most relevant consideration here.
I agree that this feature is buggy and not very UX/UI friendly. I would also argue that most people with advanced needs for their clipboard already use a (way more powerful) dedicated clipboard manager (like Clipboard or qCopy). There are probably not many users anyway and they are probably power users that can easily migrate to any of the non-buggy tools. To reduce development pileup, *I vote that this should be dropped*. Plasma already has the reputation of being buggy, let us not live up to it. Side note: This would be an amazing use case for both metrics as well as surveys. How many users have this feature enabled? What do they do? Send those who have voluteered for surveys a questionnaire, etc. It is somewhat disheartening that we gather all this data, but don't use it in those cases. Cf. https://blog.davidedmundson.co.uk/blog/metrics-in-kde-are-they-useful/