I'll use a Ubuntu release announcement to show my point.
https://fridge.ubuntu.com/2020/04/23/ubuntu-20-04-lts-focal-fossa-released/
Ubuntu 20.04 LTS was the 2020-April release of Ubuntu; it was released in 2020-April (year.month) being the format of Ubuntu releases and support is covered in the announcement.
Maintenance updates will be provided for 5 years for Ubuntu Desktop,
Ubuntu Server, Ubuntu Cloud and Ubuntu Core. All the remaining
flavours will be supported for 3 years. Additional security support is
available with ESM (Extended Security Maintenance).
ie. Ubuntu 20.04 LTS had 5 years from initial release; flavors of Ubuntu had 3 years from initial release which includes of course Kubuntu.
Ubuntu 20.04.3 is a re-spin of the original 20.04 system (not a new release) with patches applied, and the remaining support that existed at initial launch ie.
https://fridge.ubuntu.com/2021/08/27/ubuntu-20-04-3-lts-released/
if you scan it, you'll find the exact same wording; but it's referring to the time from initial release.
This Thursday the release of Ubuntu 20.04.4 will be released; it'll no doubt be almost the same wording too (if you read the announcements I'm referring to, they may have been posted by me, but I don't write them).
Packages found in 'main' have 5 years of support; KDE or Kubuntu are found in 'universe' with shorter supported life; so you can continue to use a Kubuntu 20.04 LTS system after Kubuntu has ended its support; but you'll only receive security fixes for the packages that have 5 years of support (i.e. only parts of your system).
What applies here to Kubuntu, applies to all flavors, e.g. I'm involved heavily with Lubuntu, and it's the same for our packages/support.