It's been a couple days since I last posted about this, but I still haven't managed to make the Signpost plugin work in my own game. However, I have made several discoveries since.
1. I reinstalled an entirely clean (new) version of 20.1 and followed the instructions provided in the .txt file as I added the resource to that. That worked, and the game accepted the plugin.
2. I frequently make back-ups of my game, primarily before adding big things like new plugins to it. I recently successfully installed a plugin called DiegoWT's Starter Selection (
https://eeveeexpo.com/resources/747/ ), so I tried to install this Signpost plugin on the newest version of the game that did not have DiegoWT's Starter Selection installed. That also worked, and this version also accepted the Signpost plugin.
3. With that in mind, I extracted a clean version of this Location Signposts plugin from the .zip-file, re-read the .txt file to make sure I understood properly what to do, ported over the plugin just as I had done with the previous two examples and... nothing. The game is not registering that this plugin exists, or it chooses to ignore it, and since it was possible to install on versions that don't have the other scripts, something makes me think that the two scripts - despite not being related - somehow would cancel each other out.
4. And for good measure, just to see if my theory was correct, I tried installing the other script (the Starter Selection script) onto the backup of my game that got a working Signpost plugin. Just like I expected, the game fully accepted the Signpost plugin but ignored the Starter Selection plugin's existence entirely.
The two somehow won't co-exist, despite how they should have nothing to do with each other.
I will try and install more plugins to see if it's localized to just these two... or if it's a wider problem