/etc/ is a system directory so anything in it will be erased early in the install process. I mentioned I often make a copy of fstab and hosts in that directory because those are files I commonly modify on my systems.
I tend to not copy & ‘restore’ (copy-back) the files, just paste the lines in I need … but I’m installing/re-installing systems regularly for QA-test purposes; so viewing the results of blkid & pasting what is required in fstab I’d find easy & thus my preference to re-create the files on the newly installed system (I can diff what I created with the copy-pre-install to check I’ve done it right).
We all make mistakes though (why we need backups!). I did a upgrade via re-install of one of my own (non-test) 18.04 boxes replacing it with 20.04 as a QA-test (~32 hours ago), but didn’t notice the box didn’t have internet connected; so the restoration of additional packages could not occur 
The install was good so test completed 
, I just had to manually add packages myself b/c of my error
This install is a little messy, an error on completion is to be expected if you’re using any 3rd party packages, why Lubuntu suggests a fresh or clean install. I personally like it & use it a lot. (it’s rare for me to use 3rd party packages except as snaps)
For boxes where I provide support (ie. I have a box with 20.04, 21.04, impish [21.10] on it) which also has files I don’t want to have to backup & restore… I don’t upgrade the packages on those systems - but constantly freshen their packages using this upgrade via re-install method (where daily ISOs are spun, ie. focal or impish). My files don’t get touched & if I need to use that box for support or other; I know I can have clementine start playing my music without any setup even after a re-install (I use clementine by choice, but also mention it as it’s not included with a default Lubuntu install so it’s likely the first I’ll notice if packages fail to get added-back).