When functionality like this changes or gets removed, it’s usually due to security or access-control considerations. In this case, banner topics are now gated so that topics in read-restricted categories can’t be used as banners anymore.
That said, we’re currently discussing internally whether there are better workflows or improvement that could help with use cases like yours.
In the meantime, one possible workaround, if the goal is simply to avoid banner topics being visible in normal topic lists, is to create the banner topic as Unlisted from the start in a public category instead of a private one. Unlisted topics won’t appear in topic lists or category views for regular users, but they can still be accessed via direct link and used programmatically:
To keep things organized, you could also add a tag like #banner so it’s easy to locate those topics later.
From there, you can use the Discourse Automation plugin and run the “Make Banner Topic” script, referencing the ID of the unlisted topic. That will convert it into a banner while keeping it effectively hidden from normal browsing.