This persistent popup described here: https://www.dropboxforum.com/t5/Apps-and-Installations/Dropbox-software-needs-installation-on-every-startup/td-p/284927 (but I can't post into that thread) still persists into the latest Long Term Support version.
None of the answers in the old thread work for me, and haven't worked over multiple Ubuntu versions for years.
When you set Dropbox to run on startup, the command it applies is "dropbox start -i". If you go to System Tools>Startup Applications>Dropbox and edit the command to read "dropbox start", the recurring, annoying, pop-up goes away.