There are as many opinions on what constitutes Simple Haskell as there are people who have them. We don't want to endorse any one view, but promote the general idea. Here are some general heuristics to use in assessing things.

Accessibility Commercial software is a team endeavor. Fancy Haskell is costly to teams because it usually takes more time to understand and limits the pool of people who can effectively contribute.

Maturity Things that have been around longer will be more well-tested and understood by a larger group of people. Prefer tried and true techniques over the latest shiny library or language feature. The more foundational something is in your tech stack, the more conservative you should be about adopting new versions or approaches to that thing.