Friend and former colleague David W. Keith, responded on Facebook to my post about typing the degree symbol:

For temperatures there are U+2109 ℉ degree fahrenheit, U+2103 ℃ degree celsius, and U+212A K kelvin sign (no degree symbol, common mistake). Good fonts will make a ligature, but most don’t.

I was completely unaware that these dedicated temperature unit symbols existed.

The codes David mentions are for typing Unicode characters. Most folks don’t know about them. Even fewer know that you can enter them on macOS by holding Option while typing a four-character hexadecimal code. Along with temperature units, you can enter otherwise untypeable characters, like fractions ¼, ½, ¾ (U+00BC, U+00BD, U+00BE), the true multiplication symbol × (U+00D7) to complement the division symbol ÷ (Option-/), or the always fun and useful interrobang ⁈ (U+2048).

To do this, you’ll first need to add “Unicode Hex Input” as an Input Source in Apple Menu  > Settings > Keyboard. Follow Apple’s instructions to get there, then click the + button to add a new Input Source. Search for “hex,” select the entry, then click Add, then Done. Switch to that Input Source (you did add the Keyboard Viewer to your menubar, right⁈), then hold Option and type a four-character code (for example, Option-2-0-4-8 for the interrobang). Wikipedia has a sizable (but necessarily incomplete) list of Unicode characters.

(Worth noting that the Apple logo exists as a Unicode character (U+F8FF), and can also be typed on most Mac systems by pressing Option-Shift-K. However, it’s a Private Use Area Unicode character, undefined on most non-Apple systems, where it will likely display as a white box with a black outline.)