There's already the same feature for the network: In the applet and settings it shows the last time you used a connection. This would also be helpful for Bluetooth devices as it would allow you to determine devices you haven't used for ages to drop them. Also it would help you to find the right device to connect to, which you've recently used but don't remember its name.
Hi, I started working on this feature here https://invent.kde.org/plasma/bluedevil/-/merge_requests/239