well, development of ROS 6 seems to have stopped by end of 2021, which explains why there is no need for new bugfixes for the long-term: because no new features have been added, most bugs were already fixed in the latest 6.48.6. But it is still fully supported, from the public info available.
As you say, ROS 7 has major changes compared to ROS6, even in terms of performance, showing that there was heavy development involved (maybe almost starting from a blank sheet).
Well, let’s hope we’ll see a long-term ROS7 published in the near future…