This is not a regression because that option was intentionally removed because it was a refuse.
The issue is that it should not have been there in the first place. When we did some code refactoring, we reused the code for managing the tabs in the the central area also for the tabs in the sidebar. I understand it kind of worked in some cases, however, we do not officially support having multiple columns of tabs in the sidebar and we recently noticed it that it was possible (because something else was broken) and remove it.
By “officially support it”, I mean that when we design the theme or the user interactions we have to keep in mind that that UI setup is possible, which we never did.
So, I kindly ask you to open a feature request for “Supporting multiple columns of tab groups in the sidebars”
Thanks!