Set the QGIS display to show decimal degrees?

Within the Project Properties > General dialogue box, you should have an option to set the Canvas Units:

However: it seems for using Decimal Degrees as your display units, your map must be projected 'on the fly' to WGS 84 / 4326:

Keeping your map in your original coordinates seems to limit the use of decimal degrees, while you can still use DMS, Degrees Minutes, etc.

(I am not allowed to comment that's why I am writing it here.)

I had the same problem in QGIS 3.2.3 as DonA had. The solution of DPSSpatial did not work for me at first. Only when I closed QGIS, started it again with a new project, and then set the menu to the following (which is basically the same as DPSSpatial described):

Then I loaded the data I wanted it and it worked.