Looks like settings in ACP have changed since last release as @baris pointed out
Unfortunately the nodebb-plugin-quickstart
that serves as example/documentation wasn't enough to have the ACP completely working. Settings are no longer empty when I reload that page, but if I navigate somewhere else and then go back to the plugin settings they are empty. No idea why.
I also found this: https://community.nodebb.org/topic/16958/how-do-i-create-a-settings-page-for-my-plugin
But it again points out to the quickstart plugin and that plugin throws errors, it doesn't work in NodeBB 3.1.6
So I could not fix this issue, which is pretty annoying considering how basic this is for a plugin. That's all the time I had, sorry.