So there is a very big inconsistency in the UPGRADE guides of Symfony. Back in the days where BC was not a Symfony thing, UPGRADE guides mentioned all breaking changes. When BC layers where introduced, deprecations weren't mentioned in the UPGRADE guides and only the removed features were added to the UPGRADE-3.0 guide (e.g. 8da6129).

Somewhere around 2.4, deprecations were added to the UPGRADE-2.4 file and the UPGRADE-3.0 file didn't mention anything about the removed feature.

Now, the UPGRADE-3.0 file is far from complete. I think we should take care that every removed feature is mentioned in the UPGRADE-3.0 file, maybe linking to the older UPGRADE files for the full migration details.