If you are using XKit, this does indeed more or less work. It gives you a reply button back at the bottom of original posts by the people you follow, and the reply you enter gets added to the notes on the post like the old feature did.
Possible shortcomings:
It looks as if you only get to enter one reply. The behavior of the original reply feature where you could enter multiple replies does not appear to be supported. Whichever reply you enter first is the one you get.
I’m guessing that this relies on Tumblr maintaining support for the display of replies in the notes. Since they currently support that for the display of legacy replies (that is, those made before the Great Upfuckery of Reply Removal), it seems possible they’ll maintain that going forward.
On the other paw, there’s nothing stopping them from breaking the feature tomorrow, an act that would be consistent with @staff‘s established M.O.
This is only useful on the desktop. I’m not sure if there’s a version of this available via the XKit mobile app, which I haven’t used in a while due to its slower performance compared to the otherwise-much-derided Tumblr mobile app. Maybe I should try the XKit mobile app again?
Anyway, there it is. I will now be going back to being an annoyingly prolific replier to the original posts of those I follow. Thanks, @new-xkit-extension!