It is slower than we'd like it to be but the breakneck pace of the first couple of years with a new hero ever 3 weeks wouldn't have made sense in the long term either. It was necessary early on to fill out the roster but what the game needs at this stage is regular balance patches and reworks.
Development has certainly slowed down but it's still alive with new content, Hero reworks and balance patches.
https://heroespatchnotes.com/patch/summary.html