Looking at the links on that page, the only links for 9.0 or 10.0 look to be for the network edition (non oss)

I'm not familiar with the situation, but reading through https://blog.zimbra.com/2020/05/is-zimbra-open-source-yes-fa... suggests that they do still provide source for at least some portions of 9.x, but they no longer provide binaries or packages.

It looks like some components may be missing ("Modern UI"), but I don't know if it's usable without them.

9.0 onwards never has a “community edition” release so people self-hosting that rather than paying for licensing for “network edition” have stuck at 8.x. Note that there are two downloads for 8.8.15 on the linked page, one of which is marked “open source”. Anyone who is still there has been getting security updates, but that stops being the case once 8.8.15 hits EOL.