I took my Trek e-bike (Bosch motor and battery) in for a warranty repair, the mechanics lost the key. It took them an age to get a new one from Germany (height of the first C-19 lockdown) when it arrived it came from a company called Keys Direct (I think) and not Abus, that may have been a workaround on the shop's part, I'm not sure. I didn't ask. All they needed was the key serial code to replace it. I was expecting a new Abus key, but it was a generic supplier with a different plastic handle to the original, which leads me to believe, so long as you have the key code, you can get the key made by any company that can use Abus's codes . If someone nicks a Bosch system bike from a rack to sell it on, they will be able to replace the key chamber and equip a new key set, any legit shop would be asking questions at this point, which enough of a deterrent for dopey idiots, but I suspect the organised gangs that target high value bikes will not be perturbed. It's not hard to charge these modern batteries on the bike anyway, so I'm doubtful a lack of keys will put off thieves.
In case you want more security, the Bosch Kiox display can be set up to lock the motor once removed, and no other Kiox display would allow the motor to start, it's like a car key, just don't lose the display, because you'll need a new motor to get it running. A legit Bosch dealer would demand evidence that you are the owner of the bike before they replace the motor and order a replacement display. This will all add to the cost of putting a stolen bike back on the market. But... that doesn't mean whoever robs your ride knows about these systems, once you bike is gone, it doesn't matter what systems you have in place, you'll likely never see it again anyway. I like that I can take the Kiox off, and stick it in a zipped pocket, so I've no chance of losing it, and I know no one can ride my bike off very easily, and it wouldn't be easy to sell either as the motor would be useless. As thieves grow accustomed to the Bosch system, it's less likely they would bother with stealing it, but I suspect it's no bullet proof solution.