Regarding eARC, the Q90R review on HD Guru stated: Many of the "features" of the new HDMI 2.1 interface are expected eventually to be supported by the television, but nothing definitive can be announced about this now, as the HDMI LA hasn’t yet signed off on the necessary testing and certification protocols for these capabilities on the set. However, it’s our understanding that this television will eventually be updated to support such key features as eARC, ALLM, Freesync 2 and Variable Refresh Rate, but it’s strictly “buyer beware” until the proper certifications are issued and announced.
Some have speculated on this thread that the lack of an announcement on HDMI 2.1 features support is a sign of bad news, the theory being that if Samsung planned to support it, they would be eager to announce it. I'm not sure it really works that way. As noted above, a manufacturer needs to go through certification steps with the HDMI Licensing Administrator (LA) before they can announce support. There are likely strict rules dictated by the HDMI LA regarding what can be announced and when. So the lack of any official announcement may very well be just a matter of these steps not yet being complete, and not an indication either way that support is coming or not coming. But I think the fact that Samsung is pursuing certification with the HDMI LA is good news, indicating their intent to support these features.
Still unknown though whether or not they will be successful in obtaining certification, so I agree "buyer beware"! For my part, I am going to wait a little longer, thinking there is a very good chance that they will end up supporting eARC, but I'm not buying until they announce it.
I also wonder why they are so late in going through the certification process? It could be because issues are being discovered during certification. It could also be because they just recently got the TV through product development and into the certification process.