Initially introduced in 2017 by 4fcbcf4e4880717fc3cb4099fdfa634c26c5b977 (and
follow up commits) and ec7d81f67c844f692113d756db2bd328f92bd554
See https://bugzilla.gnome.org/show_bug.cgi?id=773341 and
https://bugzilla.gnome.org/show_bug.cgi?id=792693 for more details.
This was made to support the legacy behaviour of adaptive demuxers (regarding
streams added/removed dynamically).
Since then, a lot of things have changed in decodebin3 and related elements
regarding how dynamic streams are handled and this custom behaviour is no longer
needed.
This also removes weird behaviours like EOS being delayed until *all* streams
were EOS, which could cause deadlocks downstream.
Fixes#3666
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7194>