Back in the day engines were simpler and accessible; Haynes manuals were fantastic! Are they still popular today?
Yes, they are still popular in the hobbyist community for many cars made before a certain date. The manuals are highly accessible in the US because the public library system generally hosts the digital copies, so if you have a library card in most states, you can access the manuals online. Help support your public library!
It makes sense that the people who built the car would have more details.
With that said, as somebody who occasionally does some work on his own cars, I’ve found them very useful for every car I’ve ever owned.