This is generally true on the server side. But, thankfully they allow you to store your files on a WebDAV server. I’ve been doing that since Zotero first started and I’ve got items that were added back in 2006.
I know, not perfect for everyone, but it avoids paying for a monthly storage bill.
WebDAV support is nice to save money, but from a privacy perspective it’s a huge bummer that the sync servers get all your citation metadata. A better self-hosting story¹ is one path to resolving this. End-to-end encryption² similar to e.g. Firefox Sync is another. Zotero has a security overview³ that shows they clearly care about good practices, but it’s still bothersome to have to trust the server when many other applications have proven E2EE works great even for non-technical users⁴.
Unfortunately from the main Zotero dev’s responses, it seems clear that they have no incentive to implement either and probably never will (look, the same comment from 2½ years ago⁵) without some shift in circumstances (massive increase in funding, new regulatory requirements). Even if a community member implemented the entirety of either solution, dstillman can just (rightly, tbh) claim it will increase their maintenance burden when they are trying to support paying customers.
I know, not perfect for everyone, but it avoids paying for a monthly storage bill.