The app Insight for Atlassian Cloud disappeared from the Marketplace recently, why would this happen? We can’t install the app anymore, but another instance with the app previously installed doesn’t have any problem.
In the same FAQ that @ibuchanan mentioned you can read about the future of the app:
As of March 31, 2021 there will be no new sales of this app.
One year later, on March 31, 2022, the app will reach its end-of-life and be switched off. Users will need to have migrated their data from the app into the integrated version of Insight for Jira Service Management Premium or Enterprise by this date.
There is a migration section if you would like to read more on that.
With regards to differences in functionality the Cloud integration will have the majority of the most commonly used features of the Insight Add-on and more.