The current Debian LTS release comes with Qt 5.11.3 and the current Ubuntu 20.4 LTS with Qt 5.12.8. So even if people run the latest Debian-based LTS OS, they won’t be able to contribute code until 2022 (unless they install PPAs or the MaintenanceTool provided by the Qt Company).
I personally don’t mind as I code by myself with 5.15, but other people are relying on the the official repositories and won’t be able to compile the code anymore.