After each version of Ubuntu has reached its end-of-life time, its repositories are removed from the main Ubuntu servers and consequently the mirrors.[365] Older versions of Ubuntu repositories and releases can be found on the old Ubuntu releases website.[366][367]
From the wikipedia page https://en.wikipedia.org/wiki/Ubuntu_version_history#Version_end_of_life
Do older Ubuntu releases completely lose access after EOL?
Until some days i was using Lubuntu 16.04 and i had acces to the "older" repositories that they granted me with the instalation. I think that what they do is delete the official repository, but the mirrors are maintained alive. The problem may be if the version that you want to use is too old, you will have problems to get the lasts versions of some program(i.e firefox), and that would be a bit risky. You will no recieve security actualizations, but you can use the EOL version(I suggest an LTS version, because they have more repositories and mirrors)