For datepicker and multiselects, we depend on deprecated jquery.ui modules, which should be removed (T49145, T100270).
In the case of multiselects, the module is not part of Mediawiki core, but rather is included in directly in the CentralNotice codebase. It has not been updated in a while.
A thorough overhaul of the CentralNotice Admin UI is, I think, desirable... Still, in any new UI, we'll still need date selection and multiselect widgets. So, in theory, it seems much of any new code written to use OOUI for these widgets could be re-used in an rewritten Admin UI.