Not sure what's the issue with the use of a cement board is.... unless supporting glass tiles, or wrong thickness so it is not matching drywall...

But unused electrical outlets with no wires... just take them out... push with a flat headed screwdriver and let them fall inside of the walls. Cut a drywall to close the opening. 10 minutes total.

Your picture is odd. It seems to show new work electrical box... that is not attached to a stud. The right side seems like 1.5" away from the stud the electrical box above is attached. Could this have been installed by previous homeowners.?

Just put a matching cover plate over the empty electrical boxes.

You have other, better things to occupy your time and energy.

It looks like there are wires running through that box. Normally, one put put a blank plate on it. But since there is no junction in the junction box, you may be able to cover it up. I defer to real electricians.

Are you sure the switches aren't wired to some plugs?

Guess I forgot to mention, they had plates over them, the modular style for coax, cat 5 type plates that hadn't been knocked out. They had to have been added during electrical rough in since they are attached to the studs and the wiring runs through the box, they are also coated in drywall mud and primer. I'll eventually pull the switch box and see if that gives a better visual field otherwise I'll go with something along the lines Mark is thinking.

It's tough to tell from the photo what the cables in that box are, thanks in part to the spray painting. But from their diameters, the two outer ones might be RG59U coax, and the one on the left of the right knockouts might be Cat5 or Cat6 Ethernet.

Look in the basement or a utility closet for a place where these cables might terminate.

If your house is pre-wired for CAT5, that's really nice. While everything uses WiFi these days, in urban environments there tends to be a lot of interference. You spend a lot less time diagnosing and fixing networking problems, if you hardwire all the non-mobile devices in your household (e.g. your game console and you video streaming devices).

You might also find that you need more than one access point to adequately cover your entire house with WiFi signal. If the noise floor is really low (such as in rural settings), you can get away with a single powerful access point. If the noise floor is higher, you do much better with multiple access points that you all turn down as much as possible (that encourages devices to properly roam from one device to the next).

So, yes, if you find ethernet wiring in your walls, hire an electrician for a few hours work and ask them to properly terminate all of these locations and put proper ethernet jacks into the walls.

Will you have the property inspected before purchase? The inspector might be able to provide some information about these junction boxes.

I like M's idea but wondering if Ethernet jacks high up on the wall like that will be very convenient and aesthetically acceptable with wires coming out of them. But should be easy to drop down to elec. outlet elevation if desired. Just something to consider.

wondering if Ethernet jacks high up on the wall like that will be very convenient and aesthetically acceptable

Good point, but it depends on where they're located. At a kitchen counter, they'd make perfect sense.

This appears to be standard low voltage prewire matching plates you removed. Near room switches they could be for controls, alarm, volume, smart controls etc.

Finally getting back to this, unpacking never ends, yes the property was inspected, but the inspector did not remove the plates to see what was behind them.

Today I went to see if I could read the wire markings and just kept pulling, the smaller gauge wires are a set of red and black "speaker" type so 2 of those for each box, not terminated just dangling in the wall. The thicker gauge is a 4 strand red, black, white and green, again not terminated and just dangling in the wall, but those may be a clue as to the massive bundle of 4 strand coming out of a plate in another wall.

We've pretty much come to the conclusion a copy of the as-builts and a cooler weather trip to the attic may be the only way to solve what the intent was here.

I understand about speaker wire. That can potentially be useful.

But it is disappointing that they put four conductor "thermostat" wire into the walls. That is so much more limited in what you can do with it. CAT5e or better yet CAT6a would have been so much more useful.