If I recall correctly, they placed IPs in an array as a temporary measure.

This was mentioned in passing however IP bans were able to be implemented shortly thereafter without resorting to this. I will repeat, as stated above, there is no personally identifying information of users in the site’s source code.

Another worry I have is that, with private modules, it'll become only "source-available".

This isn’t necessarily true. While the majority of the site would remain available under the same OSS license, probably only a few hundred lines of code (out of hundreds of thousands) will be in private modules. And these private modules will likely be completely proprietary and closed source, though not crucial to the site’s function (similar to a plugin). Although the Hype CMS is primarily focused on in-house use and not designed for ease of use for end users (we probably won’t help you set up your own instance). I wasn’t entirely sure why it was decided to be open source besides plain transparency with the community and possible archival but since it is I have no plans to revoke that.

If we want the site to stay open for any newbie Laravel developers to use as a reference…

If you are looking for another site to learn from as a Laravel dev, I recommend the osu! website. It is also open source and available on GitHub.