Your system tells you a newer upgrade is available; yes can delay that (where you’ll be asked again), or dismiss it permanently (putting all need to upgrade on you given the system was told not to nag you). These options apply to all of the Ubuntu family (ie. Lubuntu isn’t any different).
The Lubuntu team also provide EOL notices on this discouse
on our blog site
with the blog post propagating to Planet Ubuntu and other RSS/news feeds, so you can find that in many places.
Standard releases have a 9 month supported life; so 20.10 in it’s name told you 2020-October release, and even without notices you can add 9 months to that and get 2021-July is EOL. Releases are usually the 3rd or 4th Thursday in a month, but mid-late in the month so it’s pretty easy to guess (21.10 is scheduled for 14-Oct; ie. 2nd thursday so you’ll risk being at most a week out).
If you don’t like release-upgrading that often; use the LTS or long-term-support releases, as instead of months, you have years to upgrade.
Lubuntu 20.04 LTS doesn’t need upgrading yet; and the first offer won’t occur until after Lubuntu 22.04.1 has come out in the second half of next year; and then you’ll have months after to upgrade.
ie. we offer two upgrade paths; 20.10 was a non-LTS so was on the 6-9 month release-upgrade cycle; installing a LTS release means the release-upgrade cycle is 2-3 years for upgrades.
A final fallback is to upgrade via re-install, a feature I really love about Ubuntu desktop systems (works with flavors like Lubuntu too). It’s a Lubuntu QA-testcase; and is thus mentioned in Testing Checklist - understanding the testcases where the testcase is titled Install using existing partition. You’ll find details on release-upgrade there too, as we QA-test that; but as you’re upgrading to a release version; the -d option we use in development testing doesn’t apply to your case.
We (Lubuntu) don’t control the archives; no date for the move is given except it occurs after a release goes EOL. For 17.04 it was the next day; for most releases it’s much later. If you don’t want that hassle; upgrade before your release goes EOL & source changes are not required. If you are still using a release when we send out the final aforementioned EOL notices, you need to act rather quickly but that was back in July; thus your upgrade cycle became more complicated.