Looks like we used to have a widths prop for <ToolbarItem /> that was commented out in a core bump (and later removed) - #10022. Digging into that, it looks like it was because the CSS variables used with that prop were removed accidentally (https://github.com/patternfly/patternfly/pull/6057/files), which probably broke the core bump, so that feature was commented out. Then we added the vars back in core (https://github.com/patternfly/patternfly/pull/6302/files) and never added the widths prop back.

Also there is a .pf-m-flex-grow[on-{breakpoint}] modifier in core that can go on <ToolbarItem /> and <ToolbarGroup /> that we should add a react prop for.