Moving to engineering, I'll delegate decision of which version to Antonio Larrosa or Fabian Vogt

Comment from engineering:

Advantages:

Latest fixes and features, not just important bug and security fixes
Qt 5.15(.2) is the last (non-commercial) release of Qt 5, so pretty much every Qt 5 application will be compatible for years to come, while 5.12 is too old for many

Disadvantages:

Qt 5.15 is not supported upstream by Qt for some time, so we'd be moving from officially supported software (5.12 is LTS) to EOL software
It breaks HiDPI autodetection (https://bugzilla.opensuse.org/show_bug.cgi?id=1185048#c17) (but we'd have to work around or fix that anyway)
5.12 -> 5.15 is ABI compatible, but deprecated quite a few functions. Software building with -Werror like YaST needed changes in TW due to this

Comment from Engineering:

I'm in favor of upgrading to Qt 5.15. Note that although it's true that it's not supported upstream, this fact is in part compensated by it being supported upstream by KDE (as announced in https://dot.kde.org/2021/04/06/announcing-kdes-qt-5-patch-collection and explained in https://community.kde.org/Qt5PatchCollection ).

Is this not covered by https://code.opensuse.org/leap/features/issue/9 ?

It is not, since KDE Plasma is not part of SUSE Linux Enterprise.

I've had just closed a quite a lot of duplicates requests to update to 5.15.2

I did have discussion with Antonio about this:

yes, the idea is to update Qt to 5.15 in SP4 and Plasma to the latest LTS version (which I guess it's 5.18)

6:25
the problem is that we're currently blocked by https://bugzilla.opensuse.org/show_bug.cgi?id=1185048 (or https://bugreports.qt.io/browse/QTBUG-93454 which is the same bug upstream)
6:25

Fabian made a patch that seems to work, and I'm in favor of using it even if that means having a patch not approved by upstream

Lubos Kocman 6:25 PM
Oky, so shall I update tickes with this info?
6:25
Works for me
6:26
If it works at least until there is upstream fix

The next LTS will be Plasma 5.24, barring any late changes to schedule a new Plasma release.

cc: @alarrosa

Even in TW the Qt version is a bit behind (5.15.7 is current one if I'm not mistaken)

Qt 5.15.2 is the last freely accessible version of Qt from the Qt Company. KDE is maintaining a patch set on top of Qt 5.15.2 for KDE Plasma.

Last update from Antonio

the idea is to update Qt to 5.15 in SP4 and Plasma to the latest LTS version (which I guess it's 5.18)

the problem is that we're currently blocked by https://bugzilla.opensuse.org/show_bug.cgi?id=1185048 (or https://bugreports.qt.io/browse/QTBUG-93454 which is the same bug upstream)

Fabian made a patch that seems to work, and I'm in favor of using it even if that means having a patch not approved by upstream

Antonio shared news with me about KDE 5.24.0 being LTS but also that it will be released shortly after SLE 15 SP4 feature drop deadline which is on 2nd Feb 2022. Nevertheless we've received green from SLE Product and Release Management, therefore we'll have it in.