Only in use in discover and plasma-workspace it seems.

According to https://community.kde.org/Frameworks/Policies

In addition, a Qt LTS release remains supported until the next Qt release after the next Qt LTS release. For instance, when Qt 5.15 is released, Qt 5.12 remains supported until Qt 5.16, to give time for people to migrate from Qt 5.12 LTS to Qt 5.15 LTS. When Qt 5.16 is released, both Qt 5.12 LTS and Qt 5.13 are dropped, to go back to "last 3 minor Qt releases".

That means we would support 5.12 until the end? Can we still mark it as deprecated sometime?

Used by upcoming KMyMoney code as well. Is there an alternative already which I am not arware of?

As per the api docs you can QConcatenateTablesProxyModel starting with Qt 5.13

In T12276#212417, @davidre wrote:

According to https://community.kde.org/Frameworks/Policies

In addition, a Qt LTS release remains supported until the next Qt release after the next Qt LTS release. For instance, when Qt 5.15 is released, Qt 5.12 remains supported until Qt 5.16, to give time for people to migrate from Qt 5.12 LTS to Qt 5.15 LTS. When Qt 5.16 is released, both Qt 5.12 LTS and Qt 5.13 are dropped, to go back to "last 3 minor Qt releases".

That means we would support 5.12 until the end? Can we still mark it as deprecated sometime?

I think the idea was that we continue with the minimum Qt version bumps as if Qt would be continuing with its releases after 5.15 as well, so that end of 2020 we'd reach 5.13.