Dropbox has quietly made a big change to its free, basic plan.
Dropbox users on the free plan can now have a three-device limit on the number of devices that can be linked to an account. This tidbit was quietly added to a Dropbox support page which also says that the change was made in March 2019.
If you're already on the free Dropbox plan and you have more than three devices linked to your account, all of your currently linked devices will stay linked. However, you won't be able to add any more linked devices.
You'll need to sign up for a Plus or Professional account if you want to link more than three devices to your Dropbox account. Those subscriptions cost $9.99 per month and $19.99 per month and include 1TB and 2TB of storage, respectively, compared to the 2GB of space that free users get.
Dropbox hasn't made any announcements about the change, so we don't have an official statement on why it was made. It seems likely that Dropbox is trying to push users toward a paid plan, as it views customers who want to link more than three devices to their account as power users.