The letter I received was a form letter which leads me to believe others were impacted, too:

We periodically review our partnerships and titles made available through our online learning platform, to ensure appropriate diversification, topic coverage, and audience fit. The learning platform customer base is rapidly evolving and growing, which means the learning resources we offer our customers must as well.

I'm writing today to let you know that while we have greatly appreciated your ongoing partnership with O’Reilly we have made the decision to not renew the contract for the distribution of XXXX content in the O’Reilly learning platform effective XXXX.