When working to maintain a large open source project with hundreds of monthly contributors, in which those forks/branches contain a significant number of commits themselves, squashing is preferable when merging to master. We only really care about the end result of a Pull Request (in the context of Github), and we use that to assemble changelogs and release notes.

There isn't a one-size-fits-all doctrine here. It's subjective based on the goals, size, contributor base, etc of an individual project and the leadership of the project.