where is the piano roll editor in 4?

My understanding is that Mark McKay had been working on porting these features a while back, pre-release of 4.0. I do not see any projects on the Git Hub development for this (searching piano roll). Is there a reason why this is not being integrated, given the apparent effort that has been put into it? I can only assume that there is reluctance due to the focus on the play engine and the play from notation controls that may be in conflict. But can't there be a play from notation engine "off" or piano roll / MIDI timecode transport mode "on" feature to get this stuff integrated apart from the great, fancy new sounds that we have (and they are very great and very fancy indeed).
Backward incompatibility of files and incomplete data transfer via .xml makes it unworkable to merely use both 3.6 AND 4 in parallel. Without reverse compatibility of files, doesn't it make sense to 'sandbox' the features from the new play engine so that work using those tools can be done in the same environment? If integration of new and old features is a problem, then can't the integration be worked on down the line while the features stand separate for now? It's no big deal to revert to msbasic sounds while working piano roll or doing transport sync. I know lots of people want these features directly integrated to the play from notation engine for many good reasons, but does that mean that the fundamental use of the features can't be made available with limitation until full integration is worked out?

We are very much lacking assurances on these issues. There is only hearsay, optimistic speculation (frankly without warrant or evidence), or very old posts on several features with no assurances from folks "in the know". Assurances are welcome. Mods, if you can't give an assurance of future implementation (and work being done), then could you put this request on the radar of someone that can respond?