Hi Tim,This is a good idea which if you want to add to our request to enhancement portal, the link is found here: https://www.ibm.com/support/pages/node/6208986
One solution other than reviewing all the layout tabs can be to use the org export capability. I think its tedious but just so you have options. The steps are:
1. Navigate to Administrator Settings -> Organization2. Export the organization data, including layout information3. Unzip the .resz file and view the json file using a json viewer tool such as https://jsoneditoronline.org 4. Among all the json sections is 'layouts'. If you search for your custom field, it will show you which tab layouts reference it.