Sorry, it's hard to balance the competing objectives of stability and enhancement, but we try to stick to the rule of limiting changes to the "stable" (i.e. 6.4) release to bug fixes and minor enhancements. The latter category is a bit vague but generally it is based on how much code is upset by the enhancement. When it's more than a trivial amount (as it was in this case), my inclination is to favor stability. Particularly since for those who prefer enhancements, the development version is available for use without restriction.
So I would suggest that if you want to take advantage of that feature, download the 6.5 version and start your testing. (I don't recommend updating directly from the stable to development versions in a production environment without testing. What I do recommend is that developers use them themselves as beta testers for the latest development version, and when they are comfortable with it, move it into production.)
On a related note, while we haven't set a date for advancing the cycle and moving the 6.5 development version into the new 6.6 stable version, it will probably happen in the first quarter of 2019.
