The monotonic relationship between adapt_delta and the leapfrog integrator step size is unchanged by this update, and so increasing adapt_delta will by construction reduce the step size and potentially the number of divergences (i.e. if it reduced divergences before, it will reduce them now). Remember that conditioned on the same step size nothing changes here – the adapted step size will just be a bit larger.
If you want to verify this empirically then please go ahead, but given the upcoming 2.21 release I will start pushing hard if this can’t be accomplished by next week.
Any changes to the algorithm will change the exact results of a model fit. The recent addition of more checks changed the results seen by the interfaces, and this will too. The changes will just be more visible in the presence of a problematic model. Because the qualitative user experience doesn’t change, however, the authority remains on the algorithm side.