Yeah, it's come a long way since then ;) It's 100% plug and play, at least with fedora/ubuntu/debian. Haven't played much outside that realm.
Is it still necessary to research whether a piece of hardware is supported on Linux, and how well it's supported, before buying the hardware?
That said, I'd probably check out a laptop in general if I were going to buy one to see if anyone has major problems. I was bitten about 5 years ago with one where the touchpad was too sensitive, no matter what I did.
But if you want to save yourself potential trouble, it's a good idea to research beforehand to make sure there's no obvious compatibility issues.
Also, these days various vendors sell machines with linux pre-installed so that's an option.
Best general advice I can give is to stay far, far away from anything with switchable graphics, and if possible anything that requires proprietary drivers.
I have one of those machines. It's from Dell. Hooking up an external monitor doesn't just work. The scaling across the Ubuntu UI is all messed up.
Otherwise, multi-monitor setup is plug-and-play. I even have an anecdote, where Thinkpad with Fedora saved a meeting ;)
Because at least for Linux, its desktop support is still severely lacking. Maybe it's the distribution I was using (Ubuntu), but all i wanted to do was hook my laptop up to my monitors and use the external monitors alongside my laptop monitor, like I do on Mac or Windows laptops.
I failed.
I remember hand coding an XFree86Config file almost two decades ago to get three monitors setup.
So yeah, that's why I don't work with Linux on the desktop. Because it doesn't work for me on the desktop.