Good question! Given how straight forward the core requirements are (text files and forums) it really could be something FOSS. Hell you could probably do it well enough on a lemmy instance.
What UG has is years of data and the work of people spotting errors in tabs.
We can certainly make it happen but it is a ton of work and it needs to be by people with training and or years of music experience willing to donate their work.
True, though Open Source when it comes to "art" is a bit weirder. As others have said maybe Crowd Sourced is the right term, and keeping the content as something like creative commons would be the most open way to go.
I am happy to help with this on the code front but I am not at a skill level where I can write tabs at all.