They decided to keep just the System Monitor, so System Activity (a.k.a. KSysGuard) wasn’t ported to Qt6.
If you are comfortable compiling programs, I’ve been using this port:
Be sure to clone, or download, the kf6 branch (which is the one linked above).
I really tried sticking with System Monitor, but it doesn’t seem to list all processes, as I asked this other day on another thread:
Also, the newer one resets previous customizations on the process list every time you try to edit it, for example to add a new column, and I can’t make column reordering work.