Robert in CA wrote:
I tried another Microsoft 4000 keyboard I had that I replaced
because the spacebar is sluggish after I tried to clean it (big
mistake). Anyways the backspace key still did not function on
the 780. I didn't want to try this on the 8500. We can keep restoring
further back but what if they all are the same and I've lost the
back space function? Is there no way to recover it?

I did a search on the subject and these came up:

Solution 1: Turn off the Sticky Keys and the Filter Keys feature
1. Type ease in the search box from Start. Then click Ease of
Access keyboard settings.
2. Make sure the status of Sticky Keys and Filter Keys are all set
to Off. If you see On, switch to Off.
3. Your backspace key should work now.

https://postimg.cc/y3yqkpCb

https://postimg.cc/zL2GfBFg

I noticed that only the 'turn on toggle keys by holding down the NUM
LOCK, or SCROLL LOCK was switched ON. Could this be the problem?
Sometimes the driver may be corrupted or outdated, and then the
connection between the keyboard and your operating system is corrupted,
leading to the issue – Backspace, Spacebar or Enter key not working. ...
Next, click Uninstall and then restart your PC to check if Backspace,
Spacebar or Enter key could work.Dec 31, 2020

I don't understand his solution, if you uninstall the keyboard how will the
computer detect it when you re-start it? It has to have drivers to function.

1. Open the Hardware and Devices troubleshooter by clicking the Start
button, and then clicking Control Panel.
2. In the search box, enter troubleshooter, then select Troubleshooting.
3. Under Hardware and Sound, select Configure a device.
https://postimg.cc/G48jh8CW

I didn't start anything not without your saying I should.

Maybe I should try buying a new keyboard just to make sure?

I keep getting Malwarebytes updates free notifications. I think you explained it
to me once. If you click yes they stop until the trial runs out and then you have
to click yes again until it runs out again or buy premium outright, correct? It's a
lousy marketing scheme to annoy you into buying something.

Thoughts/suggestions
Robert

I've seen the advice about Sticky Keys and Filter Keys,
but there's never any root cause, any authoritative evidence
for it.

It's well worth turning them off, as a test.

https://download.lenovo.com/km/media...0108031137.PNG

*******

We know it's not hardware, because you plug in a different
keyboard and the same thing happens. That says it's software.

There is not likely to be a "new driver" for the keyboard.
It's a generic driver for the HID device, using files
that are as old as the hills. If installing a "new driver"
is to do anything, it would be a side effect, of perhaps
causing the filter driver to be removed (filter drivers
sit in the stack of some device and don't talk directly
to hardware).

But generally, I would assign a very very low probability
to an "Update driver" attempt from Device Manager doing
anything. First off, it'll tell you "you already have
the most recent driver", so it's not even going to be
tempted to change a thing. How do you get that status
to change ?

Maybe deleting the entire ENUM registry key would work.
I've tested that on Windows 10 and it still works. That
causes all the drivers to reinstall. It's a way to
clean all the stacks, forget about all previous USB devices
and so on.

At this point, I want to keep the experiments simple,
to reduce collateral damage. And Sticky Keys and Filter Keys OFF
is a simple experiment to try.

If something is "watching" the keyboard, then even using
the ENUM trick isn't going to help, because once all
the devices have drivers again, the watcher will be armed
and waiting, to eat the backspace key.

Paul