Some keyboard like cherry g80-8200 and g80-8113 have very long pcbs that are not plate mounted and the effect is really noticeable towards the middle. Most plate mounted keyboards are similar, since the plate and the PCB are braced against eachother through the switches and very difficult to bend (due to high moment of inertia). If you take the plate / pcb sandwhich out of the case you will see it is very difficult to bend.
Now one factor that a lot of people ignore is the mounting in the case. Some keyboards like skeldon/ kmac/ etc. the plate is bolted to the frame and the pcb is suspended from it. This allows for a bit more give, though not much. Another variation (lightsaver, orion, etc. It's very old but the fist I know of is the oneproduct poker case) the pate is free floating and the pcb is screwed to the case(which is like a tub). I have found that this is more rigid, and it makes sense, as the pcb is tightly coupled to the case which is very difficult to move and also to the plate, so it's quite rigid.
How this affects feel: Some people can feel it more than others. I feel spring weight and slider design more than this because I don't bottom out very hard. If you are coming off heavy keyboards like IBM or rubber dome, to a lighter design (cherry reds, etc) then you might bottom out more, and if you do then having a more forgiving design might be more comfortable.
Some keyboards feature just a PCB bolted to a case underneath, sort of like a lightsaver design but with no plate above the switches. The first I know of is the oneproduct poker case. Original poker is just PCB mount and the case was a lower aluminium plate (very thick, since the screws went into it) and it was glued into a tub to make the case. For several years I had one just bolted to the aluminium slab and as expected it was quite rigid.
Some keyboards feature an acrylic plate, One of the first I was hhkb-mx from korea (
https://geekhack.org/index.php?topic=39699.msg781001#msg781001 ), then I think there were some OTD that had a partial (just modifiers) or full acrylic plate. I feel like these try to bridge the gap between rigid metal plate and a plateless design, but in practice I wonder how effective they are as most of them have the PCB still bolted to the aluminium case.