What's worse is people copycat this "baseline", and if you try to argue sanity, they just say "sure it's what Chrome/Firefox recommend" without knowing or caring that they are only supporting months old browsers now. It's madness.
Can't be too careful.
Chrome 109 is the last version to support Windows 8.
There are a lot of people on systems even older than that.
But internet-connected devices are a special case which deserves consideration.
Unfortunately Operating Systems, and the software that runs on them is imperfect. People are spending a lot of energy finding, and creating exploits based on, those imperfections.
An old car can be used almost infinitum, if there's any danger it's likely to be to itself, or at worst very local to others.
However an exploited old machine on a LAN, is dangerous to the entire LAN. And beyond.
There are hardware iterations that create a specific cut-off, but I don't think Windows 10 was one. (As i recall, if it could run Windows 8 it could run Windows 10 AND the upgrade was/is free.)
Yes there is -really- old hardware out there running XP, even DOS, but I'd argue its not something you should expose to the Internet, even as a client.
Running old browers, and related software, to browse the modern Web, is a very risky game with a very large penalty for failure.
In that context I'm happy to build sites targeting "current software".
If it is e-waste you are worried about you can always use Linux to keep your older machines useful as that both allows them to run better than Windows with a lightweight DE while also using the latest versions of the available software with all the security fixes.
You also get to have more control of how your computer will behave too.
This is also why XPs complete ESR support ended in 2019. What I just said about people also applies to certain organizations like governments, who'd rather pay through the nose for that ESR support than upgrade.
That's just the reality of it all. There's a reason why Android is so extremely pushy with installing updates to Chrome et al. in the background (and why Androids old OS depreciation process is so slow) - users just flat out don't update their software or their hardware if they don't feel the need to.
EOL from a company means nothing. People will absolutely stick on years old hardware if they can help it as long as it works for them, and you're going to still be the one on the hook unless it's truly ancient tech. (We're currently at the point where if someone uses Vista, you could probably tell them to update? Keep in mind that Vista got EOLed in 2016. Windows 7 is kinda heading in that direction, but I'm not seeing that happen until another 2 to 3 years have passed at least).