The window board appears to be square edged, which I find a bit odd. I would have thought that on a pre-WWII house (other than one with obvious Art Deco or perhaps Arts and Crafts influences) it would normally be either bull nosed or profile-edged Thus leads me to suspect that the window board has been replaced at some point in the past possibly due to rot or woodworm, or as part of the conversion from sash windows to PVCu double glazing
In the style of window where the wall is thinned at the window opening, such as yours appears to be, it is not uncommon to find a small timber frame and panel section between the skirting board and the bottom of the window board in grander, more public rooms such as a reception room on the ground floor (there might also have been fold-out window shutters hidden in this niche beneath frame and panel doors at one time). In less public rooms this frame and panel detail might well be omitted on grounds of cost with just a plain timber board fitted instead, although with the window board being most likely a later replacement, it is also possible that original frame and panel work has been replaced by a simpler, cheaper solution. When it comes to plastic window firms nothing surprises me any longer
So probably not temporary, and not to speed up construction, but certainly cost cutting. This style of window seem to be found more in middle class homes from mid-Victorian up to Edwardian period, dying out as cavity walls came in in the 1920s onwards
What to do with them? Possibly strip off the paper, sand them off and repaint to match the skirting boards, but you can't plaster over them. It is possible to rip them out, fit a small timber stud frame and board the recess with PB, then plaster. Were it mine I'd try to ascertain if the ground floor has frame and panel work in a similar place and make copies (but then I'm a wood butcher and I like working on old stuff)