Since Blender 2.8 I cannot seem to texture only 1 face. I have done this before in Blender 2.79, but now it seems to have changed completely. Is this still possible in Blender 2.8, or do I have to switch to Texture Painting?
In order to put a texture on one face of a model you have to
select that face and unwrap it or alternatively scale the existing uv map so that that specific face matches the image
add a material
add the image as a texture to the material
either change the image from repeating to clipping and overlay it ontop of the existing node tree using a mix shader or mixrgb node or create a new material and a new material slot, add the image to the material and apply it to the single face in edit mode.
(Without any specific information it is hard to give a more convenient set of steps for doing it.)
I don't know how it worked in Blender 2.79. But if you want to add texture to a single face you have to go to Edit-Mode and then mark your face. Create a new material and klick on assign. (The Materials Pannel in 2.8 is on the right side of your Viewport.)
Now you should only have the marked face painted, you are able to add textures by pressing: Use Nodes (also in the Material Panel).
I hope my description works for you, if not let me know.