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Unintentional Period Piece - Making references to contemporary people or events that didn't stay famous past that time.
Get Thee to a Nunnery / Have a Gay Old Time - Terminology that meant a certain thing decades or centuries ago doesn't mean the same thing now.
Genius Bonus - In some cases. Like, if only a history buff today will understand the reference.
Ripped from the Headlines - Could apply in some cases. There was an episode of Seven Days (1998) that used the Waco Siege (1993)
as a Whole-Plot Reference. Someone who has a detailed knowledge about the Waco Siege would get all the sly references, whereas something that happened almost 30 years ago is simply going to fade from the public consciousness and most modern viewers won't see it the same way.
Ripped from the Headlines gets the closest to what I need.
I've been re-reading The Guns of the South and I realised in 5th page it is spilled out that the Big Bad is, well, bad. Problem is, the reference to it (him having a mug with AWB
logo and the acronym itself) 30 years ago was obvious to everyone, given it was a hot subject related to the end of Apartheid in South Africa... but would be an utter mystery without the context of the zeitgeist of the early 90s and "Funny Foreigner with a Dutch-like accent" character.
Oh, and it wasn't intended to be a Genius Bonus for sure, because when the book came out, it really was an obvious thing.
Edited by Tropiarz

Is there any particular trope where some trivia or an off-hand remark, obvious at the time of writing, might be completely lost to the audience a few decades/centuries later? Sort of like with Aluminum Christmas Trees, except it's more about something becoming obscure over the years, rather than sounding ridiculous.