Yup, the widget API’s days are numbered. We are already working to reduce their usage in core.
Right now we are actively working on converting the header. We already took care of the search and user-menu dropdowns, and are now moving on to the rest of the header. Current plan is for this change to be behind a feature flag, so that we can introduce deprecations and start the migration process for the (many!) themes and plugins which are currently using decorateWidget(). @isaac has an early draft PR here.
We have also made a start on converting the topic page. At the moment we are starting with some hyper-focussed conversions of areas which are not commonly customized. For example, conversion of the ‘topic map’ is currently in progress - @kelv has a first step in this PR.
For new user interfaces, I would not recommend using widgets. However, we can’t really deprecate the entire system until we have a clear path forward for all kinds of customizations. So I’m afraid the answer right now is ‘it depends’.
It’s a similar story with the ‘raw hbs’ implementation of the topic list. It will be updated, but we don’t have a concrete plan/timeline yet.
Now that the Ember 5 upgrade is almost behind us, removing widgets and raw-hbs is the main focus for our ‘modernising JS’ team (currently @cvx @isaac @kelv and me), so we hope to make a lot of progress over the next 6 months. Any big/risky changes will of course be accompanied by an announcements topic and guidelines for updating themes/plugins.