It seems to be free now anyway, since 2005 according to the git history, under a 3-clause BSD license.
Contrary to common understanding LLVM wasn't the very first one, ACK also not, there are others predating it when diving into compiler literature.
2025: https://news.ycombinator.com/item?id=42833638
2020: https://news.ycombinator.com/item?id=22310987 and https://news.ycombinator.com/item?id=22612420
But - the repository is not "everything you need"; it actually relies on a lot from an existing platform - GCC, Lua, Make, Python etc. So, you would typically use this to cross-compile it seems.
That is not very surprising since Tannenbaum is a professor there and cowrote wrote the ACK and wrote Minix.