It's true that these don't come with a keyboard shortcut, but you can add your own!
Open System Settings > Keyboard > Keyboard Shortcuts > App Shortcuts and then add a new shortcut to All Applications, with the menu title being "Move Window to (Left|Right) Side of Screen". After doing this, in any app that's left the system-standard Window menu in tact (in my experience, the vast majority), your specified key shortcuts will split the frontmost window left or right.
You're right I should have acknowledged you can make it a keyboard shortcut.
That is what is frustrating about macOS window management, they could add everything I want as it is sort of already present. It is just missing customisation and a few additional options.
It seems that since Apple started trying to create consistency between macOS and iPadOS features they have (purposely) forgotten that people work with macOS quite differently to iPadOS :(
Open System Settings > Keyboard > Keyboard Shortcuts > App Shortcuts and then add a new shortcut to All Applications, with the menu title being "Move Window to (Left|Right) Side of Screen". After doing this, in any app that's left the system-standard Window menu in tact (in my experience, the vast majority), your specified key shortcuts will split the frontmost window left or right.