While we deprecated C3P0 connection pool as part of Confluence 7.14 release, we removed this permanently now from Confluence under 8.6.0+. For now, the only public document around C3P0 being deprecated is - https://confluence.atlassian.com/doc/confluence-7-14-upgrade-notes-1063176413.html and this does not talks about in which confluence version, C3p0 will be removed. https://confluence.atlassian.com/doc/end-of-support-announcements-for-confluence-210239673.html does not cover C3P0 removal as well.

This becomes even more important, since present supported LTS versions i.e Confluence 8.5 and Confluence 9.2 exhibit different behaviour around this. While C3P0 is still supported under Confluence 8.5, it does not work with Confluence 9.2 as C3P0 respective support has been removed. In a scenario where Confluence 8.5 is running with C3P0 settings, post upgrade to Confluence 9.2, it may likely report "Caused by: java.lang.IllegalStateException: DB connection pool is exhausted; obtaining a new DB connection for this request is disallowed." issue as Confluence 9.2 uses Hikari connection pool which by default would come up with 10 connections.

As there is no public document around it's removal, so it will be great if we can add information around it.