I made a notice on this site found here and gave an example of how you can see the effects, ie.
Support status summary of '755-lubu':
You have 359 packages (18.9%) supported until April 2021 (Community - 3y)
You have 1324 packages (69.9%) supported until April 2023 (Canonical - 5y)
You have 2 packages (0.1%) supported until April 2021 (Canonical - 3y)
You have 3 packages (0.2%) that can not/no-longer be downloaded
You have 207 packages (10.9%) that are unsupported
Your Hardware Enablement Stack (HWE) is supported until April 2023.
Run with --show-unsupported, --show-supported or --show-all to see more details
taken from a system of mine.
It shows 1324 packages will continue receiving updates, however 361 packages will not, plus a further 210 that are unsupported. Extra commands are given so I can explore this further (showing packages that will/will-not receive updates).
For some of the unsupported packages there is no concern as they’re only wallpapers from prior Ubuntu releases (including other flavors). (I like wallpapers)
Your system will update it’s software packages exactly the same today, as it will a month, or a year from now (same commands), you’ll just not receive any updates for the packages losing support.
Yes you can save all updates locally, but they won’t disappear if they’re in Ubuntu repositories. I won’t explain how, there are many questions on askubuntu & like sites telling you how.
Some PPAs maybe cleaned out when EOL is reached (when support ends, some teams are very diligent in doing that), so those 3rd party packages may disappear and local saves maybe beneficial if needed, but I wouldn’t worry about the Ubuntu’s repositories.