Cannot stop Dropbox starting on system startup

I appreciate you don't support Mint although it's a derivative of Ubuntu. Dropbox works nicely in Mint, I've used it for a few years. Maybe the part I am complaining about is Mint specific.

I just did an advanced reinstall although some of the files mentioned are not in the Mint version i.e. nautilus-dropbox,  /var/lib/dropbox, /etc/apt/source.d/dropbox. There was no change in this behaviour.

There's an article I found here which has a hack workaround in the answer, basically stopping Dropbox from re-writing the startup file by changing permissions to make it unwritable

https://askubuntu.com/questions/1039986/unable-to-disable-dropboxs-autostart-in-ubuntu-18-04

Note that this user is running your supported version Ubuntu 18.04 (which is the distro my Mint 19 is based on). So you may get this reported sometime soon by a Ubuntu user, although many of them won't notice or care about it automatically starting at boot. So I guess whether you follow this up will be down to how keen you are to debug minor Ubuntu issues.