Despite being metric, Celsius is barely used with a prefix when describing temps high enough to warrant it

[Edited because of weird auto-formatting. Edit 2 added more pedantry. Edit 3+ is because I lost the plot and had to bring it back.]

Because the SI unit for temperature is the Kelvin, which has already been stated. It has also been mentioned that K and °C are the same but with different offsets. It has not been mentioned that °C is to K as Degrees Fahrenheit (°F) is to Rankine ( R). It would be similarly inappropriate to say "millidegrees Fahrenheit" or "kilofahrenheit". I have no idea if mR or kR would be appropriate, though.

I would offer that there are two ways to look at SI ("metric") prefixes, and these can be thought of similarly with the multipliers they represent: as a prefix to the unit, by definition; or as a suffix to the value. Let me illustrate with an example.

38,000 K could be expressed 38 kK, or "thirty-eight kiloKelvin". It could also be spoken "thirty-eight thousand Kelvin" (or Kelvins, idfk). This isn't normally important for the layperson, but suppose you have a temperature meter (and, literally, I do not mean "thermometer") that has only 4 digits of resolution. 38.00 k ("38,00 k" for the Europeans?) would be how it reads out the value in question. This would be 38 kK, certainly, due to the position of the decimal.

Now suppose that temperature meter read out in °C. 38.00 k °C would, in fact, denote "thirty-eight thousand degrees Celsius" for the reasons mentioned above.

So, because Degrees Celsius is not an SI unit, in the technical sense...

Btw, I have been explicitly using upper case letters when spelling out the units. This is incorrect. The symbols for SI (International System of Units) units should be capitalized when they respect a person (K, A). The names of the units should be all lower case because you are not naming the person, but the unit named after them (kelvin after Lord Kelvin, and ampere after Andre-Marie Ampere).

Yeah, I know. I'm being pedantic. It's literally my job. I really should be sleeping right now. Here's a source: https://www.bipm.org/en/measurement-units/si-base-units