Well at this point it is not even clearly disclosed, as I have just downloaded a release .tar.gz from github, unpacked it, ran it and saw no mention of telemetry or its opt-out options.

The free software vs. open-source distinction is quite subjective but irrelevant here - Powershell source code is released under the MIT license which allows anyone to produce a modified telemetry-free build but sensibly does not obligate the authors to provide one themselves, hence my original comment.