So installation_repositories event is fired on repo getting added/removed in a GitHub App installation (as documented on https://developer.github.com/v3/activity/events/types/#installationrepositoriesevent).
However I noticed that, when the GitHub app is installed on an organization, this event is only triggered when such action is performed by org administrator rather than org member (but a repo admin).
Here’s the step-by-step repro
Create a GitHub app (I created https://github.com/apps/installationrepositorieseventtest) that asks for Repository Metadata: Read Only & Repository Content: Read & Write (Permission does not matter here though but list for completeness).
Install this app by an org member (but a repo admin) onto an organization, and select 1 repo s/he has admin access to
Check the app ‘Advanced’ page and we can see ‘installation’ event fine (although it’s installed by non org-admin)
Go to the app install again (For me, that is https://github.com/apps/installationrepositorieseventtest) and select an additional repo and click ‘Update access’ (See screenshot at bottom)
Expected to see ‘Installation_Repositories’ event but it’s never fired (If you do step #4 as org administrator you will)
Screenshot for step #4