Things changed after WWDC and marking indexed items as publicly indexable will not make them available globally at this time. So the only way to get publicly indexable content currently is to support web markup.
Is this still the case today that the only way to get publicly indexable content is supporting web markup?
The search documentation guide at https://developer.apple.com/library/archive/documentation/General/Conceptual/AppSearch/RevisionHistory.html#//apple_ref/doc/uid/TP40016308-CH99-SW1 was last updated in 2016 and it only has a short mention of needing the public indexing property AND web markup. The public indexing property reference https://developer.apple.com/documentation/foundation/nsuseractivity/1414701-eligibleforpublicindexing?language=objc doesn't specifically mention requiring web markup to make something public.