Update 5/23/2023: We have a newer blog post on the subject.
We’re announcing today that the preview program enabling REST API access to on-premises mailboxes for Hybrid Exchange organizations previ...
Updated May 23, 2023
Version 5.0
Blog Post
we’ve seen low and now decreasing adoption rates
I can't help but feel like this was mostly due to the fact that whenever there were problems, support and the product group would be reluctant to assist with fixing anything because it was a "preview feature".
For example, using some API methods via the graph explorer worked, but using them outside of that didn't, due to them using slightly different backends as we discovered after a 6 month support case. We also had cases where some methods would work in one hybrid tenant but not in another, which resulted in the product group issuing a "will not fix" due to it being a preview feature.
Of course there would be low adoption if the thing was intermittently broken and there was no viable support...
For those of us who are unable to move all mailboxes to the cloud for regulatory reasons, this is a massive blow, especially with more vendors developing only against the Graph API and not EWS.