Yup, it got fixed on some devices, but the Nexus 5, for example, never got the fix.
This leads to the problem that you, as a dev, either have to leave a huge amount of users without working push notifications, or use GCM anyway.
This leads to the problem that you, as a dev, either have to leave a huge amount of users without working push notifications, or use GCM anyway.
This was fixed at https://android-review.googlesource.com/#/c/221708/ and probably merged into many custom roms generated since. It also only occurred under fairly specific circumstances.
EDIT: Yup, https://romhut.com/versions/cm-13-0-20160508-v8-0 indicates it's been in CM for a while, which is probably why I haven't experienced it. Dianne Hackborn fixed this as far back as November 2015, it might have made it into 6.0.1 on some devices too, though I could be misreading.