I don't think it's "bad"; it's damn simple and works well for what it is, and contains no unnecessary complexity. No dependencies on some other huge framework, multilayered overengineering, or excessive generality. The same can't be said of the many other clones of it that were attempting to be "better designed" rewrites.
As for "maintainability" or all that other software engineering stuff: this board script doesn't really need to be maintained, because it works, and if anything needs to be changed, it's so simple that the changes can be made easily. Along the same ideas as http://suckless.org/philosophy
(that's their board for cat pictures, the most SFW one I could find, but the banner ads are probably still NSFW, so keep that in mind)
I think you're thinking of tinyboard: https://github.com/savetheinternet/Tinyboard