Plasma 5.24 plans to require Qt 5.15.2. It is scheduled for release Feb 8 2022 https://community.kde.org/Schedules/Plasma_5#Future_releases We also expect distros to use the KDE Qt 5 patch collection and are unlikely to support distros which do not. https://community.kde.org/Qt5PatchCollection It'll also require the latest KDE Frameworks planned to be 5.91.
We have also scheduled dates for Plasma 5.25 and Plasma 5.26 during 2022. These are likely to happen but we will switch to Plasma 6 development at some time as yet unknown when Frameworks 6 becomes good to develop against. The final Plasma 5 release will get long term support status. There's no other LTS releases planned. Jonathan