Did the permanent CPU throttling, even when cooled off again get fixed eventually?

That's not style, that's practicality. I carry my laptop more often than I upgrade its RAM so I want it to be compact, 12 inch preferably. If you want more then go for T series and leave X series subnotebooks alone.

Is that good one to buy?

I bought a ThinkPad X280 i7-8550 16G 512SSD two months ago, I would like to say that it is really a good laptop, stable system and good battery, I think it would be around 8 hours if I only use it for work.
The last laptop I use is ThinkPad X230 which I bought in 2012, it is still working but a bit slow now, I think it would be a fast laptop if I add it with a ssd.
I have many important documents so I do not want my laptop have problems, ThinkPad is the only choice to me ...

Well if all else fails, be willing to pay the tad risky premium (warranty or the lack of it I mean) and deal with the slightly heftier weight of 51nb/lcdfans' Thinkpad X210 instead.

From what I can discern from the field reports, seems like a worthwhile gamble.

What's the KRO on this? Aka n-key rollover? On my T470S if I type fast enough it'll miss keys if three were pressed in a too-rapid succession, which is an issue I've not had in previous macbooks/dells/everything else. Seems the n-key rollover is a cheap 2/3 on this model. What about the X1?

Google thinkpad n-key rollover for many others with this issue, I'm shocked at lack of mention in reviews.

Really wish it had a 13.3" screen - I would tolerate downsizing from 14" to 13.3" in return for a smaller form factor, but 12.5" is probably further than I'd want to go (especially with a 16:9 aspect ratio display - screen height is quite shallow).

The chassis has the space too - the X1 Carbon, and at lower price-point, the T480s show Lenovo are fully capable of slimmer bezels (nevermind the competition), so a bit disappointing.

Even thinkpad is soldering RAM now. I get it if it's LPDDR (for those who wonder LPDDR is not the same as DDR-L) but only ddr is just sad. Any other advantage? No, you still get small battery because they don't even bother to shrink PCB after saving space from soldering.

We are in the era of expensive RAM yet they are limiting upgrade option even more.

Hmmmmm style over functionality? Definitely put me off from getting rid of my X230. I mean, not being able to upgrade RAM, sounds like what Apple did with their Macbook Pro Retinas. Bit silly if you ask me, and it's no wonder people are now soughting after The X230, X220 and perhaps the X240. The ultrabook thing, personally, should die off.