Hyphenation of a compound modifier formed of an adjective and a noun
"The software is open source. Open-source software is distributed with few restrictions."
While I agree with you that "The software is open source" doesn't look quite right yet, opensource.org uses it exactly as you have above:
"Introduction: Open source doesn't just mean access to the source code. The distribution terms of open-source software must comply with the following criteria," etc.
From: https://opensource.org/osd-annotated
Over time, I think we will come to accept "open source" as used in your first example.