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7:34 AM
Since deprecating another tag is currently discussed on meta maybe it is useful to check the current status of deprecated tags. You can compare this with numbers in November 2019. (There you can find links to older counts, too.)
There were 1000 questions with deprecated tags in August 2017. Now there are 872 questions with deprecated tags.
Here is breakdown by tags:
has 314 questions, 180 after removing most common algebra-related tags, 101 with no top-level tag at all.
has 166 questions, 80 without co.combinatorics, 42 with no top-level tags at all.
has 393 questions, 242 without the most frequent geometry-related tags and 144 with no top-level tags at all.
I think that one of the questions in those tags might become roomba-eligible. (The previous counts included such questions as well.)
Here are the links used to find the above numbers: chat.stackexchange.com/transcript/10243/2018/10/27
As usually, I'll remind that it would be relatively easy to prevent these tags from being added to new questions: The existing deprecated tags should be blacklisted and Blacklisting (deprecated) tag while they still exist.
There are 100396 questions in total, 78189 of them have a top-level tag, 83040 if we also count linear-algebra, graph-theory and set-theory.
This means that 22.12% of questions have no top-leve tag, it is 17.29% if we take the less-strict interpretation.
 

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