End of Support for T420s?

I already leaed, that Lenovo thinks Windows 10 is not suitable for the T420s as it's not selectable as used operating system, as well as that the T420s is seen as out of service prior to the usual 5 years after end of sales as per the matrix of Spectre and Meltdown support fixes as the T420s is not mentioned there, although in official sales until March 2013.

So, at what exact date it went out of support and why isn't it possible to find any information?

I'm using Thinkpads since 18 years now but this really annoys me as the laptop is still performing well and not being able to secure it against this threat doesn't make me feel comfortable...