We do not need individual episode tags
To start with whilst most episodes are pretty much standalone, they are all set in the same universe, share characters and reference each other. Here is what Charlie Brooker, the show's creator, has said about a connection between them.
My answer to that has changed, actually. It always used to be that it's just a bit of fun. But then sometimes we've done some things where we did explicitly refer to other episodes. I think the rule is that when a character says something that explicitly refers to something else, it's canonical. Also, they follow the same dream universe. That's the other thing that I tend to say. There's a line in 'Hated in the Nation' where someone refers to a crime that happened in 'White Bear.'
Cinema Blend, Are All Black Mirror Episodes Connected? Here's What The Creator Says
It's also worth noting that no other shows have episode tags, to my knowlegde, and they don't appear to be useful anywhere in my opinion. Creating them here would start a dangerous precedent.
Borrowing the tag scoring sheet from here (and shamelessly copying Rand's formatting from here) we get the following results.
Does it make sense to be an "expert in black-mirror-episode"?*
Maybe (people can be experts in individual episodes but as they are connected an understanding of the universe is required); score 1.
Does it make sense for a question to be tagged only with black-mirror-episode"?
No (there should also be a black-mirror tag); score -1.
Does black-mirror-episode have a single, universally-unambiguous meaning?
Yes; score 2.
Is black-mirror-episode likely to be used correctly just based on its name?
No (white-christmas and the-national-anthem are easily confused with other things); score -1.
Are there "enough" (> 15) but not "too many" (> 10% site-wide) questions that qualify for black-mirror-episode?
No (There are only 19 black-mirror questions which isn't many in itself); score -1.
Are people like to use black-mirror-episode to find questions to answer?
Maybe (e.g. if they've just watched one of the episodes); score 1.
Are there likely some users (be objective!) who will favorite or ignore black-mirror-episode?
Maybe (I doubt it, but who knows); score 1.
Could black-mirror-episode be reasonably used to feed questions to a specialized chat room?
No; score -1.
Can black-mirror-episode be used to search for questions (for any reason) in a way that keyword searching cannot accomplish?
Yes; score 2.
Total is 3, which is apparently not enough for the tag to be worth creating. From the linked question:
tags that score > 12 are "good" tags, tags that score < 8 are "terrible" tags, others are likely good but may need some clarification/renaming/etc.