After ~4 years, I'm actually shocked Apple never introduced a standalone Touch Bar keyboard. I would think if they really wanted to push that tech, they'd make it available for all notebook users who run at a desk (with external displays, KB, mouse/TP), plus all the iMacs and Mini owners.
Three substantive issues .
1.
The touch bar is driven by the T-series chip. The main application processor and GPU draw into a RAM framebuffer and the T-series copies it over and present it to the screen (and handles the touch/GUI interactions and sends that back). So essentially would need something priced akin to an Apple watch in the keyboard. ( that the other functions of the T2 can't be decoupled so you would have to duplicate the whole thing into the keyboard.)
Part of the reason for the 'copy RAM frame buffer" move is that the mobile iGPUs Apple used didn't have the DisplayPort (DP) stream to drive the touch bar to spare after provisioning the lid screen and the DP to the Thunderbolt ports.
So then you'd have to put the cost of the typical Apple keyboard on top of that. Quite doubtful few would by it
.
$99 ( Magic keyboard
https://www.apple.com/shop/product/MLA22LL/A/magic-keyboard-us-english )
$199-250 ( Apple Watch )
$299-350 Keyboard. (probably more because that is probably not a big enough battery if wireless).
( pretty much in the zone of an iPad 8th generation and sidecar. )
2. Apple doesn't do any wired keyboards . Bandwidth and battery.
Bluetooth LE isn't really designed for video signal traffic. The iPad Sidecar Touch Bar can work wireless , but the iPad has a battery much bigger than a keyboard to run the connection over full fledged Wi-Fi=point-to-point.
Wired? sure that could change things , but that somewhere Apple wasn't willing to go. 'Air' everything. ( But USB 3.1-3.2 could pump the limited video data and the keyboard traffic and draw power from the host system. )
3.
The other issue is that the Touch ID has typically gone with the Touch Bar. I suppose could unique pair and encrypt the touch ID info but not quite as secure as fixed connections internal to the system.
Sidecar is basically the expansion past the units with it built in. That isn't huge use case expansion, but bigger than zero.
Perhaps this is less "entangled" to implement now that the T2 is merged with the main processor and if they are willing to decouple the TouchID. ( and if willing to use a wire. ) . It would be simpler with as keyboard with essentially a "dumb" video monitor and USB hub to pump the data.
.