New Backdrop user here, but I've used Drupal 8 for a year or so. Looking at my modules in Backdrop there's "Property Validation" -- I don't recall installing it, so it must be included by default. It's disabled and states it requires CTools. I tried installing CTools (https://github.com/backdrop-contrib/ctools), but it failed with "master.zip does not contain any .info files." CTools' page also states there is no reason to install it any more. I did some searching, but didn't find anything related to this.
Add validation rules to properties. less Requires: Field (enabled), Field SQL Storage (enabled), Entity (enabled), Ctools (missing) Tags: Fields