The best I can think of for this combination of topic-specific-ignorance and a feeling of exclusion or having been left out or left behind is:

[to be or feel] out of the loop

From Free dictionary idioms:

Not having knowledge of or involvement in something: A few people at the top knew what was going on, but everybody else was out of the loop.

Not part of a group that is kept up-to-date with information about something.

I think this captures the sense of paranoia and of being left out that the question mentioned.

Some definitions (e.g. Google's) talk about it being ignorant of "information known to only a privileged few", and many others talk about its use in the context of decision-making - but this is usually when there's some such context provided, usually political (for example "Only the Prime Minister, the Chancellor and their closest advisors knew the full implications of the plans. Everyone else was out of the loop").

If you use it without any such narrow context, it can mean generically out of touch with common knowledge and conventions. For example, "I watched a topical comedy show and didn't understand half the jokes. I feel so out of the loop".

Search for a phrase like "I feel so out of the loop" and you'll see many such examples of it in the context of feeling lost, out of touch and topic-ignorant, rather than politically excluded.

The best I can think of for this combination of topic-specific-ignorance and a feeling of exclusion or having been left out or left behind is:

[to be or feel] out of the loop

From Free dictionary idioms:

Not having knowledge of or involvement in something: A few people at the top knew what was going on, but everybody else was out of the loop.

Not part of a group that is kept up-to-date with information about something.

I think this captures the sense of paranoia and of being left out that the question mentioned.

Some definitions (e.g. Google's) talk about it being ignorant of "information known to only a privileged few", and many others talk about its use in the context of decision-making - but this is usually when there's some such context provided, usually political (for example "Only the Prime Minister, the Chancellor and their closest advisors knew the full implications of the plans. Everyone else was out of the loop").

If you use it without any such narrow context, particularly when talking about feeling out of the loop, it can mean generically out of touch with common knowledge. For example, "I watched a topical comedy show and I didn't understand half the jokes. I feel so out of the loop".

Closely related (and maybe better in some cases):

[to be or feel] out of touch

Free dictionary idioms again:

not informed about something: The report shows that the committee is out of touch with recent developments in space technology.

knowing no news of someone or something; not keeping informed of the developments relating to someone or something

An important difference is, if you say someone is "out of touch", it's implied that they're to blame for not keeping themselves informed; if you say someone is "out of the loop", the implication is that they're ignorant due to never having been informed by others, without really saying anything about whose fault it might be that it became like this.