Due to the short period and a focus on enterprise features, there isn't a huge amount that's new in this release. We have had a lot of updates to improve strings in our ten supported localisations. We have also acted on feedback we received to improve one of our English UI strings (RM-5793).
As noted above RM 2.6.c has been tested with Alfresco 5.2.x, but we don't know of any compatibility issues with the latest community release of ACS. If you try it and find issues then we'd love to hear about it. Compatibility with 6.0.x is on our backlog, but we don't expect to address it until RM 3.0.a (nb. our next community release is currently expected to be 2.7.a).
If you have any feedback about 2.6.c, or about what you'd like to see in the next release, then please let us know. You can use the comments below, or feel free to message me or one of the team.