The base language is remarkably stable. Haskell 2010 removed (n+k) pattern matching. And that's the only thing obsoleted over 12 years !

So it's all add, no removal.

A better question to ask is, what's the 20% of the report that's used 80% of the time?

> Should I always look first in the 98 Report, and only if I don't find something there then look in the User's Guide?

Sounds like a good plan. Over time, you'll understand what's haskell and what are ghc extensions.

Humble advice: it's not enough to read the language def. You'll understand better by reading the conference proceedings that made the language what it is today.