The Mathcad 15 unary operators /°F and /°C are actually implemented in Mathcad Prime. If you convert a Mathcad 15 worksheet to Prime, those operators are converted properly and work. However, typing them in is a royal pain. They should both be on the Units drop-down tool bar.
These operators are needed to strip units off of variables for:
clear and concise use on plot markers
providing unitless values to built-in and custom functions that require a certain temperature unit (°F or °C)
quickly comparing values with affine units to equivalent values with no units from other sources
Recommendation: Put the inverse affine unit labels on the Units toolbar and mention them in the on-line help.