Under The Qt Company's plans to move Qt LTS point releases later on to be for commercial customers only , Qt 5.15.3 LTS kicked off that effort to boost their sales. Unfortunately, that left open-source Linux desktop users in the dark for those that aren't Qt commercial customers and at a time when the Qt6 support last year had a lot to be desired with numerous modules not ported over at the time and other issues.
Now as part of the KDE Free Qt Foundation , one day less than one year since Qt 5.15.3 LTS was released to commercial customers, The Qt Company has published a freely-available open-source version of that point release.
The open-source Qt 5.15.3 release was announced today with source packages being available for download and a new "v5.15.3-lts-lgpl" Git branch. That Git tag was made back in November but only pushed out publicly and announced today.
Qt 5.15.8 LTS meanwhile is their latest commercial-only release, so it looks like we'll be waiting quite a while longer before the newer open-source point releases are made available - v5.15.8 will presumably be open-source now next January.