I have a 14 year old T420, I upgraded the processor, ram, hard drive, battery, and wifi chip several years ago which really sped it up and gave it more usable life. Still runs great for most things.
With an SSD it boots insanely fast and consumes very little resources at idle. And will get security updates for many years to come. It's really the perfect OS for the T420 (or Linux of course - although I'd actually prefer Windows for a classic Thinkpad).
Bit hard to buy legally (you need to go through a VAR - there are some who will definitely sell you single copies). Core Windows 10 without all the bloat is actually a really impressive and lightweight OS considering the massive backwards compatibility and huge backlog of free software. Not to mention the insane breadth of hardware it will run on.
Still, it's fast enough to use with linux, and the keyboard is a joy to use. Swappable batteries are fun, and useful.
However, I can't really use one outside of just nostalgia, or for tuning cars.