As mentioned above, there is a free open source in-memory patch available:
https://github.com/valinet/ExplorerPatcher by a talented developer, Valentin-Gabriel Radu to enable only the Classic Taskbar without enabling "UndockingDisabled" and prevent the side effects it comes with/the things it breaks. As Windows 11 is finalized, the developer aims to update it to work with newer builds/releases.
Update: As of now it's already compatible with the latest builds.
There is also
StartAllBack, an outstanding commercial, proprietary app, priced very sensibly, from well-known developer, Tihiy who's enhancing and fixing Windows with his little projects since the Windows 9x days.
Of course, for now, both projects simply enable the old Taskbar code still left intact in the Windows 11 OS. If Microsoft decide to remove it, some developer will have to step up and develop a full taskbar alternative or reverse engineer the full classic Taskbar code to re-create it. However, currently there are no compromises with the approaches used by ExplorerPatcher or StartAllBack. All of the above features (broken from UndockingDisabled) work due to the in-memory nature of these apps. Both projects even let you disable the new dumbed down context menus. You don't have to use any of the above mentioned workarounds as new Windows 11 features still work.
ExplorerPatcher and StartAllBack bring back the full classic Taskbar while keeping full Windows 11 functionality and offer several of their own enhancements.
How to use Classic Shell / Open Shell as the Start Menu and StartAllBack/StartIsBack++ only for taskbar fixes without it interfering with the Start button of Classic Shell / Open ShellHere's a special transparent Start button specifically for the use case where you want Open Shell/Classic Shell as the Start Menu but StartAllBack for its taskbar fixes. Put this in C:\Program Files\StartAllBack\Orbs and then from StartAllBack Settings, set this transparent Start button in StartAllBack settings: Taskbar. Then in Classic Shell/Open Shell, use whatever Start button you want to use and set left click and Win key to open Classic/Open Shell Menu.