So I'll give you a few things which I have found to be useful over time:

1) GE includes a service diagram with all their appliances. May be glued to the back of the stove or tucked away in the lower drawer or the control panel - but it will be there.

2) If both elements are working properly by themselves (you can check the ohm readings, may be below 20 or even below 10 but not 0), then there's really only two possible causes - a failure in the oven control system or thermostat.

I may be wrong, but there only appears to be a conventional, mechanical thermostat/control in this oven.

So, it may be that this oven does not necessarily run the bake and broil in series (50% voltage). Not all did.

Can you unplug the stove, take the panel off the back and check out Range/Stove/Oven Temperature Control Thermostat WB21X489? Are the wires firmly attached and not loose or burnt? follow them down  to the broiler and bake elements.  Does anything smell burnt or weird? The part is still available, but pricey - nearly $300.00 with shipping. At that point, getting a used one for a 30 year old stove isn't a bad idea. 

Check the continuity on that device on all the settings. If you don't know how to do this, let us know. It's easy to learn and only takes one of the Harbor Freight meters they're always giving away or selling for less than $10.00.

This will at least get you through to knowing what is working and what may not be. Super important that the stove be disconnected for all these measurements, not just for your safety but also so you don't get false readings. A purely electro-mechanical system, it's reliable and should be fairly easy to diagnose.

And, if the bake times haven't changed then it does raise many questions.

So - no browning could be caused by a a few things besides time/temperature.

If the moisture in the oven is not escaping out the vent, that might account for it?

Have you checked that? Is it possible that there was once a reflective shield under the broiler which is no longer there or a flare up when broiling darkened it to the point it no longer works? An overly aggressive oven cleaner ate through much of it or discoloured it?

Could a rack have been lowered one notch - makes a gigantic difference or, perhaps a different set of baking pans is being used? I'm sure you'd have thought of this, but asking for the sake of completeness.

As to GE versus Whirlpool, they do often use the same parts in some lines. All original Whirlpool parts will say 'FSP' somewhere on them. Roper is a very old, very well respected oven maker which was bought by Whirlpool for their technology many years ago. 

Here's a screenshot of your service sheet - the settings for the oven temperature control.

That there is only an 'L1' and not an "L1" and an "L2" suggests a super simple control (more options with both, but irrelevant as rarely implemented).

The PL means Pilot Light, so you should have a light which comes on when heating and goes off when the temperature in the oven has been reached?

The BK is the bake element.

The BR is the broil element.

As you can see, there is never an "X" in both columns for BK AND BR at any time, just for PL and one or the other, separately.

This is confirmation that we are not working with both elements in tandem, either in series (110V) or parallel or something else.

Here are two other things to check, not that I consider either likely:

a) Could it be that your house voltage rose to 220/240V from 208V? Your utility company could tell you. It wouldn't make that big a difference in heating times, so you might not notice it, but it would definitely lead to shorter baking times - and that could result in lessened browning.

b) Is it possible that a seal between the door and the oven is gone or has failed, or the door has slightly warped, reducing the heat at the top? Or, truly unusual but trying to think out of the box, did the door or the seal get tighter, and there's more moisture in the oven or a shorter baking time?

Try this. Bake some cookies, spread out as far apart on a baking sheet from each other as possible on the rack height at which you usually bake. Do they all come out evenly in the baking pan? 

There are several people here who really know these things inside out. If we can't figure it out together, one of them will no doubt chime in at some point.

Here's another improbable to check - is it possible that the baking element was knocked out of alignment with the spacers during cleaning? It isn't supposed to touch the bottom of the oven at any point. Those spacers can sometimes get knocked loose, whether they are wire or steel pins or ceramic.

In the same light - could the thermostat bulb have come loose from its bracket within the oven? If it's in the wrong position, it might well be telling the thermostatic control that it's warmer in the oven than it really is?

I didn't know that!

Today, Roper is often the value brand for Whirlpool, like 'Estate' or 'Kirkland'.

Same quality as the equivalent other Whirlpool brands, just way cheaper.

I thought of the sensor in the oven or the heating element being knocked, didn't occur to me that the regulator might have gone out of tune.