Just over three years ago, Canonical launched the family Bionic Beaver of your operating system. It arrived in April 2018, so the main version was called Ubuntu 18.04 and the rest of the flavors added the same numbering to their respective names. Like the April releases of even-numbered years, this was an LTS version, which means they are supported for a longer time, but not all flavors are supported for five years.
The five years of support are only for the main version, that is, the one that GNOME uses and the name is simply Ubuntu. The rest, Kubuntu, Lubuntu, Xubuntu, Ubuntu MATE, Ubuntu Budgie, Ubuntu Studio and Ubuntu Kylin are supported for three years, so, already in May 2021, they have reached the end of its life cycle. The last maintenance update for them was 18.04.5, August 2020, but they kept receiving new packages and patches until last April 30.
Ubuntu 18.04 will continue to be supported. The rest will have to update
Users who are still using a flavor of Ubuntu 18.04 Bionic Beaver have to update now. Personally, I would recommend backing up your important files and perform a scratch install, since it is likely that someone is using a 32-bit version, no longer supported, and there are some flavors that have even changed the graphical environment, such as Ubuntu Studio, which went on to use KDE Plasma, and Lubuntu, which went to LXQt.
As for which version to install, if an LTS was being used, the logical thing is to think that another Long Term Support is preferred, so the jump would be to Focal Fossa (20.04). If you decide to use the most up-to-date release, that’s 04/21 which arrived just over two weeks ago. Since the one I would not upload is from Ubuntu 18.04, the main version, since Hirsute Hippo seems a more transitional version than usual, but Ubuntu will continue to enjoy support for two more years. Choose what you choose, it is a must update now.