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

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».

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...

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

Probably useful in a non-latin codeset?

having a dedicated Kelvin symbol preserves the semantics.