No. ACM has been on a slow path towards making everything Open Access for many years now - driven both by internal and external feedback. Given that the ACM Digital Library (individual and library subscriptions) has been subsidizing other ACM endeavors, that making stuff OA is not something you can take back later, and that the required article processing charges (APCs) cause other problems, they have been rather careful with the transition.

They plan to make the entire digital library freely accessible by January 1, 2026.

Here's a short article about timeline and implications: https://medium.com/sigchi/about-acm-open-cd544408559c

EDIT: for anyone wondering why there need to be APCs at all, Jonathan Aldrich gives a glimpse into costs and benefits of 'traditional' publishers: https://medium.com/sigchi/what-benefits-do-traditional-publi...

Most likely not, at least not directly. CACM and arXiv do not in any way compete.

However, arXiv has done a lot for open access publishing, so perhaps it wouldn't have happened (this early) had it not been for arXiv.

arXiv is more than twenty years old at this point. I think the rising opposition to exploitative publishers has ratcheted up pressure on other institutions to switch to an open access model more than arXiv suddenly causing policy changes.

In 2019 there was a big dustup when people thought Trump was going to issue an executive order requiring all federally-funded research to be open access. ACM as an organization signed a letter with a bunch of publishers opposing this idea. It's own membership (myself included) objected pretty strongly... and the next year ACM announced plans to move to open access. So, just maybe, they're actually listening to their members.