Smoking everywhere. For anyone who wasn't around for the 70s/80s/90s, everything was tinged yellow and smelled of smoke. Car/plane/train seats had built-in ashtrays. Restaurants had smoking sections separated from the non-smoking sections by waist-high walls.
I have asthma and it sucked. Not sure if I grew out of it as I got older or if there's just not a miasma of smoke around everywhere, but it rarely bothers me anymore.
At the risk of becoming too anti-casual, anti-gay slurs were so common in the US up until the mid/late 90s, if you weren't there for it you just have no idea. One of the Bill and Ted movies (I think the first one?) just randomly dropping it in there as a joke, where the slur is the joke, is a good example of just how it was then. There's still bigotry but it's not as casual and pervasive.
The lack of privacy, independence, and freedom that generally comes with childhood.
Women having to get husband’s permission to open a bank account (speaking of the US).
I'm glad that Furbys, inflatable furniture, and disposable cameras are no longer mainstream. And may they never return.
Medicine in general has gotten a lot better. I'm also able to buy stuff like silken tofu without having to drive quite far to find a specialty store that sells it.
Smallpox
George W. Bush's presidency. I don't know what the kids are smoking when they say current republicans make him look "classy" or that Trump's first term was worse than his. He bred a constant state of paranoia and xenophobia and used it to justify killing countless people in the middle east. The damage done to that part of the world is staggering and everyone just treats it like background noise.
Also that decade post-SpongeBob where every kid's cartoon was about a loud, annoying, dumb guy.
smoking everywhere
I’m so fucking glad we’ve stopped calling women “hysterical” whenever we don’t believe them. That word is so blatantly misogynistic and it seems to be dying out now.
Leaded gas.
Objectively, dial-up.
Otoh, what I would really badly like to become a thing again is actual media ownership, ie. not having streaming services randomly yank your stuff away from you.
Also, I would nominate the fact that the 'It's obsolete as soon as you get it in the door' meme hasn't been valid for decades now, but hardware manufacturers, Windows itself, and the game industry are trying really hard to make that a thing again seemingly.
The US.
Smoking everywhere and anywhere. Younger folks have no idea how ubiquitous it was, not to sound boomerish, but everything smelled so bad, and people would smoke in places that would shock you now, like in hospitals they would smoke in the nurses station, if you walked into a clothing store at the mall they'd be smoking at the counter, etc. Even when they did things like making that glass room for smokers at Tim Hortons, I once saw a woman sitting in there with her toddler in a stroller puffing away. It was actually amazing that anyone put a stop to public smoking because so many people did it.
Not to parrot everyone else but, Smoking for sure. The damn smell.
Another is probally lead based products. Such a health risk and was so pointless
The Hedolith.
