Interpreting Flavor Support Policy against Overall Ubuntu Release Lifespan
(This part is with my Ubuntu Developer and Lubuntu Council hats on.)
As a member of the Lubuntu Council I am fluent in this policy.
Essentially, while the core of an LTS release - Ubuntu Kernel, Main repository packages, etc. - are still supported through the LTS lifecycle of Ubuntu as well as a flavor, the Flavor-specific components (desktop environment, specific applications shipped with that Flavor but not others, etc.) are not 'supported' by that flavor's team past the 3 years. Which means you can still file bugs, etc. against those packages, but no new feature development nor any bug fixes are guaranteed to get any support from that flavor's development team (essentially becoming 'community supported' after that flavor's support timeframe).
Kernel, underlying core of Ubuntu OS, Main repository packages, etc. will still be 'supported' in the sense Security team and other teams are still going to handle bugs, updates, security patches, etc. But the flavor specific components (LXQt for Lubuntu for example) are not expected to get Lubuntu Development Team support after three years. Doesn't mean other developers and such can't fix critical bugs or do security patching, just means that it's likely to not happen after those 3 years for a flavor.
What does this mean for Ask Ubuntu though?
(THIS part is with my mod hat on)
We don't follow the flavor-specific 'support' period, we follow the overarching 5 year LTS support limit. We also qualify as 'community support'.
Therefore, using Lubuntu 20.04 LTS as an example, in 2023 the Lubuntu Desktop LXQt components will 'no longer be supported by the Lubuntu Development Team' for the purposes of bug fixing, etc. However, 20.04 still is 'supported' overall by Ask Ubuntu because it has not gone past End of Standard Support (or End of Life) as dictated on the Ubuntu wiki for the overall release. There's just no guarantee of bug fixes or such from the Lubuntu team after those 3 years.
Any flavor of Ubuntu outside of the flavor team's 'support' period is still supported by Ask Ubuntu until the overall release reaches EoSS or EoL.