Basically Apple’s mobile Photos.app interface but for browser tabs.
https://i.stack.imgur.com/lxfU9.png
Is this what you are describing?
The problem is its current implementation completely kills your SSD if you have too many tabs. The problem has been going on for years. To the point I have to manually disable Tab Overview gesture and icons.
I also found Tab Overview is a disaster if you have too many tabs. I've occasionally been looking at Google Maps and done pinch-to-zoom-out. Big mistake: Safari zoomed out of the page instead and tried to show Tab Overview. That scheduling loading every tab that was previously in a sleeping state, just to show the thumbnails, and would typically show as using 30-40GB memory while I realised I'd have to kill and restart the Safari process to get back to work.
I like Safari, and switched to it from Firefox because Firefox was getting very slow. (Turned out to be the Tree Style Tabs extension, not Firefox itself).
But it eventually I found Safari is good at seeming fast while using a huge amount of memory that slows down everything else on the system. It was not unusual to find Safari using a lot of swap and increasing over time, and being the reason other things I use for work were getting slow.
I switched back to Firefox and found, rather pleasantly, its memory use rarely shows higher than about 10GB.
I have the exact problem with pinch to zoom. I had to disable it to stop Tab Overview from loading, once ran out of memory it will start paging everything to SSD, which is why I said it is basically kill the SSD.
I still dont know why the Tab Overview wouldn't automatically become a list view once it is over certain number of tabs.