Prior to Horizon, it was easy to be in the loop for changes in a project's configuration: Either they had to go through operations/puppet's hieradata/ or through https://wikitech.wikimedia.org/wiki/Hiera:$project. This also meant that it was easy to roll back changes, or look at how classes/variables had been set in the past.

When I look at an instance in Horizon, changes to the "Puppet Configuration" tab are not listed under "Action Log" or anywhere obvious, the same for "Project Puppet" and "Prefix Puppet".

Creating a watchlist function is probably overkill :-); what might be neat is to pipe changes to adminbot/IRC, i. e. if I apply the role foo::bar to an instance, a project or a prefix, that would be transformed as !log tools scfc applied foo::bar to sna::fu on IRC which then updates the SAL on wikitech which can be watched as usual.