Indeed, I discovered this annoying loss of functionaltiy when coaching a training for CP2019 end of August. If my memory is correct (only have CP2019) it was always possible to rearrange the sequence of content master slides (not quiz master slides) in the Master slide panel. I used that to have the most used master slides on top of the list, a recommendation I often gave to trainees and during consultancy jobs. No longer possible, it is not the only functionality that has been lost. I did log a bug report, but have little hope that it ever will be fixed, since you are the first one to mention that bug in this forum. If something is only discovered by the 'unhappy few' it will never make it to the top of the priority list. New hype features that are spotted by the majority are more important, which is understandable. But it doesn't solve our frustrations.
I often find workarounds, but here it is difficult. I try to keep the number of master slides as low as possible, start for a custom theme with the bare-bone Blank theme (has only the compulsory 6 master slides) and try to set the content master slides in the most appropriate sequence for my way of working (Title master not as first, you only use it a couple of times at the most).