Effectively, they left around the end of 2016, which was the last time that anything was being accomplished at a meaningful rate.
Since then we have had a change to the reward curve (mostly a coding tweak), some other minor blockchain coding tweaks, a few UI tweaks in steemit.com, a few security patches, and the RC system. As well as total non-performance on the 2017 roadmap, and non-completion and non-delivery of SMTs which was essentially the 2018 roadmap. And finally obstructionism mixed with outright hostility on outside involvement with the development process. The security patches are important and the RC system was significant development but none of this is remotely close to two years of work for a team (and also rolled out poorly).
I do not even remotely share your confidence in the "core team" whether they have explicitly announced their departure or not.
Agree, we are "stuck in a rut" ...
I share your non-confidence that anything will be accomplished at a meaningful rate in the future ...
I wished there was someone that could convince me otherwise.