After a vanilla installation of Ubuntu 20.10 I installed Dropbox from the Ubuntu Software Center and all seemed well. The Dropbox icon appeared in the system tray.
Problem is, I can't be certain when the icon disappeared but can only surmise it occurred after changing some settings to do with desktop icons to disable the trash and Home desktop icons. These have been disabled in 20.04.2 LTS and the Dropbox Systray icon persists in that version.
I tried reversing those steps without success.
What I Have also tried
The answers in this question are all from older versions but I did try:
Installed package sudo apt-get install libappindicator1
Logged out and then logged back in
issued dropbox stop && dropbox start at the command line in terminal
without success.
Dropbox is working correctly in the background. I can access files from Home > Dropbox and amend a file, resave it and can see that the file has changed so all seems well in respect to syncing.
Whilst recognising that 20.10 is short lifed (EOL July 2020) - it is installed on my development PC - and waiting for the day 21.04 becomes available.
The main issue of course is that there are a number of Dropbox utilities provided from within the Systray icon which are not available elsewhere so the Systray icon is key in this respect.
I would like to know if anyone else has encountered this issue in recent times and information on how it can be resolved in anticipation of continuing issues after upgrading 20.10 to 21.04 when available (and subsequently 22.04LTS)
I'm on 18.04 but my icon went missing as well, tried all the solutions here on askubuntu, but nothing worked. So I went looking for alternative clients and stumbled upon maestral, which needs this gnome extension called AppIndicator and KStatusNotifierItem Support for the tray icon to show up.
This extension works for me, it works with the official client and now this tray icon (and other ones) are showing up. Maybe it will help you too!