infogulch parent
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