Which a) FLOSS b) stable (= not rolling release) c) desktop GNU/Linux distributions have a long security update support?
Of course it’s interesting which distro has the longest support, but 2nd, 3rd etc. places should be considered, too.
Debian stable versions get security updates for around 1 year after the new stable version is released. In theory this could be a long time, but you can’t know for sure, can you?
Ubuntu LTS versions get security updates for 5 years.
Any distro offers more?