Haskell is hard. Both Slackware and LFS are simple. I don't see the comparison.
LFS is even better in that it provides a ton of documentation. Slackware unfortunately lost out to the modern world. But heroic effort by Patrick.
I dunno man, a lot of people still use it! Been on it every day about 4 years now, there's a modern ARM port in development, so it might not end up going away for a long time.
That Slackware is difficult to use on the level of Gentoo or LFS I think is mostly a meme and an overstatement; it's just very old-school. It has a nice installer and a good wiki.
Many of the gripes that I hear of managing other linux systems I seldom or never have experienced on Slackware. It doesn't get in your way or flippantly change things from one release to the next. It's a rock-solid choice.
LFS/BLFS is hard to maintain as a primary OS. Gentoo is practical and easy in comparison. How many people do you know stick to LFS?
The point isn't so much about hard/simple, but jumping to the extreme instead of the pragmatic approach.