In JEPs proposed to target JDK 9 (2017/4/4), Mark Reinhold has written that JEP 291 ("Deprecate the Concurrent Mark Sweep (CMS) Garbage Collector") is one of two JEPs that "have been placed into the 'Proposed to Target' state by their owners after discussion and review". If things go well for JEP 291, it will be targeted for JDK 9.
Reinhold explains in this message why JEP 291 can still be targeted to JDK 9 at this relatively late date: "JEP 291 requires only a minuscule code change, to enable the proposed warning message to be issued. It's a JEP in the first place not because it's a risky change but to bring visibility to the plan to remove the CMS collector in the long term." As these sentences state, the JDK 9 targeted action is simply to mark the Concurrent Mark Sweep (CMS) collector as deprecated with the idea that it will be removed at some point "in the long term."
Although G1GC is the default garbage collector for JDK 9 through JEP 248, it is not always the best collector for all situations. Even the proposal to deprecate CMS acknowledges this in its "Risks and Assumptions" which states, "For some applications CMS is a very good fit and might always outperform G1."
Another recent discussion of OpenJDK jdk9-dev mailing list is titled "JEP 291: Deprecate the Concurrent Mark Sweep (CMS) Garbage Collector" contains interesting arguments for retaining CMS. Christoph Engelbert (Hazelcast) writes, "CMS+ParNew is the most commonly deployed solution and a lot of applications are optimized to the behavior of CMS." Scott Palmer writes that in his specific application, "we have found that so far the CMS collector has significantly lower maximum pause times than G1." Roman Kennke (RedHat) adds, "I'd say it's too early to talk about removing CMS. And, to be honest, I even question the move to deprecate it." Martijn Verburg (jClarity) states, "We are now constantly asked to tune G1 for customers and have found that even with our most advanced analytics (in combination with some of the common and more esoteric tuning options), we are unable to get G1 to outperform CMS for *certain* use cases. Several customers have therefore reverted to CMS and are very interested in its future (as consumers)."
This same discussion also includes reasons for deprecating CMS. Mark Reinhold's post states that JEP 291 was "posted last summer" and requests were made for a CMS maintainer, but "so far, no one has stepped up." He concludes that post, "In any case, Oracle does intend to stop maintaining CMS at some point in the not-to-distant future, and if no one ever steps up then we'll remove the code."
Jeremy Manson (Google) explains the trickiness of the current situation with G1GC and CMS:
We decided that supporting CMS in any sort of ongoing fashion should be a last resort after we try getting G1 to do what we need it to do. Our belief is that fewer collectors is better. We spent some time over the last few months coordinating with some of the folks at Oracle and experimenting to see if there were plausible ways forward with G1. We couldn't find anything obvious.
The gist of all this seems to be that many applications still depend on CMS and these applications will have a deprecation warning displayed in JDK 9. The future of the CMS garbage collector appears to be in doubt, but it would only be deprecated in JDK 9. When the CMS collector would actually be removed seems less obvious, but I assume that JDK 10 is a potential "future major release" in which CMS support could be terminated. Quoting Manson (Google) again, "The short of it is: We are still willing to contribute work to support CMS, but we want to make sure we've done our due diligence on G1 first. Our belief has been that the JDK 10 timeframe is long enough that we don't have to make this decision hurriedly."
It looks likely that Java applications using the Concurrent Mark Sweep garbage collector in JDK9 will see warning messages about the deprecation of the CMS garbage collector. When (or if) CMS won't be available at all is less obvious and depends on who is willing to continue supporting CMS.