Missing battery icon in menu bar of iMac

having only had imacs and mac minis I've never seen a battery icon in any of the macOS versions I've run

I view the battery level of my bluetooth devices by clicking the bluetooth icon and choosing the submenus from the context menu, maybe you were using a third party tool which was 32bit and is now not supported by catalina ?

Add the bluetooth icon to your menubar via the bluetooth option in system preferences.

Once you have the bluetooth icon in your menubar, click on it and hover over whatever keyboard/mouse/trackpad device you want and you'll see the battery level for each one in the popup window.

+1 for DeeperDiver's suggestion.