We only support official Ubuntu releases (that are not end of life).
then why we still need to use that tags?
Why do we keep the tag? Because we still have questions that were asked for end-of-life releases when they were alive.
That sort of leads onto why we keep the questions. There are a few schools of thought here:
I make no secret that I'm firmly in the third category. We certainly don't need to keep every question ever asked but I strongly object to nuking things, even if they technically don't apply any more. They serve as good, searchable record of approaches to fixing things.
More than anything, Stack Exchange built on the premise of making the internet a better place and I simply don't see how deleting a large portion of it every 6 months is going to do that.
And it does make new off-topic questions easy to spot.