And why can't the symbol be a regular old uppercase "K"? Who is this helping?

Unicode wants to be able to preserve round-trip re-encoding from this other standard which has separate letter-K and degree-K characters. Making these small sacrifices for compatibility is how Unicode became the defacto world standard.

The "other standard" in this case being IBM-944. (At least looking at https://www.unicode.org/versions/Unicode1.0.0/ch06.pdf p. 574 (=110 in the PDF) I only see a mapping from U+212A to that one.)

The ICU mappings files have entries for U212A in the following files:

    gb18030.ucm
    ibm-1364_P110-2007.ucm
    ibm-1390_P110-2003.ucm
    ibm-1399_P110-2003.ucm
    ibm-16684_P110-2003.ucm
    ibm-933_P110-1995.ucm
    ibm-949_P110-1999.ucm
    ibm-949_P11A-1999.ucm

I think just using uppercase Latin K is the recommendation.

But, I dunno. Why would anybody apply upper or lower case operators to a temperature measurement? It just seems like a nonsense thing to do.

Maybe not for text to be read again, but might be sensible e.g. for slug or file name generation and the like...

That’s an interesting thought.

IMO this is somewhere where if we were really doing something, we might as well go all the way and double check the relevant standards, right? The filesystem should accept some character set for use as names, and if we’re generating a name inside our program we should definitely find a character set that fits inside what the filesystem expects and that captures what we want to express… my gut says upper case Latin K would be the best pick if we needed to most portably represent Kelvin in a filename on a reasonably modern, consumer filesystem.

I wonder if you can register a domain with it in the name.

A symbol may look differently than original letter, for example N - №, € - E (Є), S - $, integral, с - ©, TM - ™, a - @, and so on.

However, those symbols doesn't have lower case variants. Moreover, lower case k means kilo-, not a «smaller Kelvin».

Although it is a prefix in that case, so we should expect to see k alone.

To maximally confuse things, I suggest we start using little k alone to resolve another annoying unit issue: let’s call 1 kilocalorie “k.”

Probably useful in a non-latin codeset?

having a dedicated Kelvin symbol preserves the semantics.