So that's what this wording means:

> Making the first 50 years of its publications and related content freely available expresses ACM’s commitment to open access publication and represents another milestone in our transition to full open access within the next five years.

( from https://www.acm.org/articles/bulletins/2022/april/50-years-b... )

I wouldn't have understood that nuance without the context given by your comment, but in my developer mind I analogize "freely available" to a "source available" license that they took on, as a step towards going open access ("free and open source") over time. I'm also happy to see that that transition seems on track as planned.