Document and define a process around EOL-ing runtimes
It seems there is no documented process for this and some sort of expected EOL date and announcements will help third party maintainers using this runtime on Flathub a lot (even if it doesn't reach everyone).
Announcement
I suggest whenever a new runtime branch is created or an old one is being EOL-ed an announcement should be made either on https://discuss.kde.org/c/announcement/9 or a mailing list that everyone can subscribe to.
This is done by Freedesktop runtime eg. https://discourse.flathub.org/t/freedesktop-sdk-23-08-0-released/5089 and GNOME https://discourse.gnome.org/t/gnome-44-10-released/20041 (GNOME releases are synced with runtime releases)
Support Policy
As per the support policy of a 6.x runtime I think having exactly two supported branches (eg. 6.6 and 6.7) at any point is a good idea. More than that puts too much burden on everyone as they have to maintain extensions and baseapps for every branch. Less than that gives applications too little option if they can't upgrade timely.
EOL Policy
The EOL policy can be something like: an older branch ie. 6.6 will be EOL-ed after 1 month has passed since 6.8 is created; 6.7 will be EOL-ed only after 1 month has passed since 6.9 is published etc.
This should give enough time to everyone to plan and act on the announcement.
Lifetime
This also gives around 10-12 months of support for each Qt 6.x branch. 6.6 was released on 10th of October 2023 and the branch was created on 17th October 2023. It would be EOL-ed after 6.8 runtime is published which should be around 25th October 2024 (+1 month)
Release schedule for
- Freedesktop runtime: https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/wikis/Releases#information-about-current-releases
- GNOME (GNOME releases are synced with runtime releases): https://release.gnome.org/calendar/