The Windows Terminal team copied the UNIX terminal "standard" of only clearing the current viewport.
Now, scrolling back "works" in the sense that it shows partial(!) output from previous commands.
Say you have a script that produces 47 screens worth of output, and what you want to see is somewhere in the middle. You can't judge this visually because the scrollbar is inaccurate and there may be other history.
Before you could just 'cls', re-run the script after a change, scroll back, and see only the updated output.
Now, if you do this you'll get 46 screens of superficially identical looking output, with 47 of the output you wanted to see appended to it. This is super confusing.
It has confused me. I've seen other admins be confused as well and copy-paste the wrong thing.
The Microsoft Terminal team debated this and decided to do the wrong thing on purpose and copy the "Linux way" to pander to Linux admins on Windows.
Meanwhile if you go back through the old newsgroups and mailing lists, the same argument was had by Linux people as well, with the same logic I just outlined. They also decided to do the wrong thing. Why? Because they were pandering to UNIX admins coming across to Linux.
Why does UNIX do this? Because of an error in the coding. That is all.
That's why the shiny new Windows Terminal is broken now, to copy an error that was already a copy of an error.
Are you saying this specifically or metaphorically? Because I suspect that there was a rationale behind that decision.
See also https://man7.org/linux/man-pages/man1/clear.1.html which explicitly states that it clears scrollback iff a terminal has a specified capability. Which windows terminal being of CRT origin should have. I think the issue here is actually an intersection of some terminal nuance and windows team who blindly copied the behavior that some linux guys were used to because this knowledge was buried under their gnome defaults.
Iow, no need to blame unix and linux, blame those who picked one (stupid) mode of many and used that as the only mode of operation.
Microsot teams are tainting the Windows developer experience trying to cater to the crowd that has been buying Apple gear for GNU/Linux development.
Busybox clear does not clear scrollback nor does it provide an option to do it. But for example on Alpine where Busybox clear is the default, the ncurses package can be installed to get the ncurses clear.