As per recent discussion on Discord, and patch to the production server block, SlickStack now better supports /wp-json/ requests (for the REST API) in WordPress when permalinks are enabled, but we had to remove the old support for non-permalinks to make this work properly.
We might try to support “both” again in the future but it would require more experimenting.
For now, 2025 onward, SlickStack effectively requires you to enable permalinks in WordPress, at least if your site needs to use the REST API which is a lot of sites these days. TLDR make sure you enable permalinks in WordPress if you are going to use /wp-json/
