There were a variety of reasons MS gave for removing the customizations ranging from not looking right with the new Apple-like UX design to making it harder to make changes without needing lots of extra testing (for various customization options that could be used). Irrelevant this is not an option in Win11 and it doesn't appear that MS is going to be bringing it back anytime soon.
If you want the left/right dockable taskbar then you'll need to use third party software. Stardock's Start11 has this ability (experimental), along with many others, if you want Win11 to look more like Win10 (or earlier). It costs money of course so you'll need to decide whether the customizations are worth it. There may be other products that can do this as well, I haven't looked.