It's explicitly mentioned in the list of default packages on Fedora Silverblue and Workstation [1], and provides commands like killall(1) and pstree(1). Therefore, it should be part of the toolbox image for RHEL 8.x too.

The toolbox image for RHEL 9.x already includes psmisc because it's pulled in as a dependency of iproute.

[1] fedora-comps commit e4ed54dfcc497fd0
https://pagure.io/fedora-comps/c/e4ed54dfcc497fd0
https://pagure.io/fedora-comps/pull-request/379