MDI has never been updated for Windows 10 or newer. Which is a shame because even if it's not the modern way, older apps exist...

Delphi / C++Builder wrote a new implementation of MDI to keep older apps working, but looking modern. (It means MDI-architected apps can use, eg, tabs for windows too while keeping the old code the same.) https://blogs.embarcadero.com/3-x-12-vcl-enhancements-in-del...