6.0.4 released at 2021-04-22, 6.1 released at 2021-04-27. Where's 6.0.5 and so on? It's not released. Debian supports its releases for 5 years, not for 5 days.
Only run Debian here, but to be clear, Debian supports releases for 1 year after the next release, so typically 3 years max.
Debian LTS is handled by a third party, non-debian org, via donations. All packages are not necessarily covered, it's not the Debian securtiy team.
This is important to understand, as LTS depends upon donations to decide what gets secuirty updates. Donors have a say. So for example, you'll see apache, openssl, php updated, but not obscure pacakges.
https://mirrors.ocf.berkeley.edu/qt/official_releases/qt/
You can find a more local mirror on the mirror list
If Debian supports its releases for longer than the upstream projects do, then that's a (perfectly valid) choice of Debian, but I'm assuming that involves maintaining those projects as well, to the standards of Debian's support policies.
Qt 6.2.4 was released at 2022-02-17 and it's the last public available 6.2 version. 6.3 was released at 2022-03-16. For paying customers they released 6.2.8 few months ago, for example, but it's not available for everyone else.
That said, of course having upstream patches available for free, as it was in the past, would even be better.