Lubuntu 18.04 is only supported by Lubuntu until April-2021.
Yes, some packages (ie. the ‘main’ repository) will receive updates until April-2023, but if you use the ubuntu-support-status command, you’ll note that covers only part of your system, with ‘universe’ packages including all our Lubuntu packages ending security updates in April 2021 (some 18.04 packages are already EOL; Ubuntu Studio 18.04 was not a LTS release with extended support being provided via PPA).
There have been many threads of this site about x86/i386/32-bit support, and where people can go come April 2021, eg. 18.04 support april 2021 , Looking for a Linux distribution with support for old hardware as the main focus - #8 by azdays15 along with many others…