I don't see myself ever searching for the plot-device that is child abuse, in particular. If want somethign sad, I would search for "tragedy". Or for something more specific, maybe "tragic backstory" (though while NU etc. uses tags like that, MD rarely has tags for anything but genres (meaning, they rarely have tags even for subgenres! much less plot elements/devices, and doubly less exact versions of a plot device)).
Normally I am all for having more tags, but they have to be meaningful in terms of discovery. Otherwise they will only dilute the tagging system, and overall make it harder for uploaders to use it properly (which results in them not using it properly, either from ignorance or simple laziness, so stuff ends up lacking tags so stuff won't show up even when ppl explicitly search for that kind of stuff), and harder for readers to grasp a titles tags at a glance (information overload).
So yes, a "tragic backstory" tag, or for ones where it is throughout the actual plot (like that slow-life isekai title where everyone abuses mc), something like "downtrodden/abused mc" (surely there is a better phrase), aka tags that includes all forms of abuse makes more sense [than a child abuse tag] imo (because they describe the story/plot, rather than professing the existance of a very specific plot-element - that could have been used in ways the searcher wasn't interested in). Both for whitelist searches and for blacklist searches.
