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That, or Small Reference Pools. Depends on what the idea is - if the authors want to shout as loud as they can "look, it's the eighties!", that's "Mister Sandman" Sequence. If it's just "if we don't put the stereotypical eighties things there, the audience won't know it's set in the eighties", that's Small Reference Pools.
Funny Flashback Haircut for hair and there is I Was Quite a Fashion Victim
Yeah, those all fit, thanks, tropers!
A flashback scene will feature characters wearing stereotypical fashion items of the era, to an exaggerated level, even if in the series' present time they just dress in clothes that might be common in different settings, such as suits, white shirts, and so on.
For instance, in The Simpsons, Dr. Hibbert has a short hairdo that doesn't really stand out, but in flashbacks he'll have jheri curls, braids or dreadlocks depending on the era.
Even Homer wears a nondescript white shirt and jeans, but is fashion-focused on flashbacks, like in That '90s Show, he wore an 8-Ball jacket over a hoodie... even though when the show actually aired in the 90ies, he had the same white shirt.
Might be a Justified Trope in that characters might care more about fashion when they're young, but still...