I agree with the fact that it is difficult to know if a key is used or not. But it's not impossible.
For us translators, it is difficult to translate without knowing the context of the use of language keys. That's why I open this topic.
If some lang keys are not used, it would be better to remove or tag them.
For "LOG_" language keys, yes it's difficult. But for the others? is it difficult too?
Maybe, we can tag the suspected lang keys as you did in the en/acp/common.php
Not with the tag "deprecated", but with another such as "suspected unused".
And, if after many checks it appears that the lang key is not used, we remove it in the next release.