Would frames have been used with glass plate negatives?
In a word, yes.
I started in the B&W graphic arts industry at the very tail end of glass plates being used in an industrial photographic environment; early to mid 1970's. We had a cleaning bay where the emulsion was scrubbed off the glass of already used glass plates. Then by hand, emulsion was tipped over the newly scrubbed super clean glass from a jug. There was one employee who was a wizard in getting a reasonably consistent emulsion coating without wasting too much liquid emulsion. I once had a go, devil of a job, but fun to try.
Once dried the plates were sometimes duped using a glass frame holder. To ensure the plates are held rigidly, we used wooden wedges around the perimeter of the glass to wedge the two glass sheets to ensure registration was kept.
These were using an orthochromatic emulsion, which was done under a red (I think) safelight. Later on and just before we stopped using glass plates, I seem to remember we used yellow light, fluorescent safelight tubes right at the end, coinciding with an emulsion supplier change. Which would be around 1975-6. The yellow light was very welcome as it meant many processes could be conducted directly under the yellow safelight in a part of the room, while normal white fluorescent tubes were in the rest of the room. I remember we were amazed for quite some time with this advance in graphic art technology.
I would suggest in a home environment, a glass frame holder is possibly not an absolute requirement, but having one would probably be beneficial to your reliability rate.