Link to GitHub
Now i've really wanted to use this plugin, although there is something theres missing which would really improve the plugin, it's also added on the "TODO" list but i haven't really found any progress on it lately.
Now I thought if it really would be hard to make a "Select as best answer" option for the person who created the thread, that would automaticly lock the thread and set it as "SOLVED"
If it really is just 100-150 old topics, then an nginx map makes even more sense.
I can see it from your point of view, that it's easy for NodeBB to do, although I urge you to consider that nginx is very good at what it does, and so having nginx check for an old url to redirect is quite simple.
For example, if the old forum had topics using this url: /t/<topic id>, then you could write a location block that only responded to that folder, and applied the map to redirect the user based on those topic IDs.