Plasma 5.15.90 was released today, see:
https://www.kde.org/announcements/plasma-5.15.90.php
It will be tested the next 3 weeks in the kde-next repo.
As always, place kde-next above all in your pacman.conf. If you do not know how to edit pacman.conf, then please don't try to attempt testing betas of Plasma, editing the pacman.conf is the easiest part of this kind of testing.

20 days later

Qt 5.13 RC is now also being added to kde-next, full rebuilds of Frameworks & Plasma are ongoing.

EDIT:
New Qt 5.13 upload delayed, changes in cmake file generation from qt5-tools breaks the build of many in KDE.

EDIT 2:
It is now uploaded, temporary workaround used in qt5-tools.