When Qt 6.0 is out, 5.15 will still have the patches on Gerrit, so if there are any issues you want to get fix, you will need to get the source for the latest 5.15 open soruce bug fix release, and then apply the patches. This is just stopping the release of further bug fixing releases.
On your Open Source project, you have 3 options:
1. you stay with the latest open source 5.15.x
2. you cherry pick changes an generate your own 5.15.y thaty ou can use,
3. you move to qt6.
I’m certain the technicallities of how the dev branch will work will be published soon.