As reported in #3526471: [5.0.0-alpha3] Menu items do not get is-active class, this MR fixes the issue for me in ui_suite_daisyui for me, after adding some extra logic in the menu component of that theme.
I think having active menu items is pretty basic expected behaviour for a theme, so would be good to get this included sooner rather than later. Having to patch both a module and theme to get a visual indicator of the active menu item, isn't the best situation for getting adoption IMHO. Therefor I took the liberty to increase the priority of this issue. Feel free to lower priority if you disagree.