You really nail the design and accessibility issue on the head. I found that after the initial roll out I have never bothered with sending stickers and other stuff. I also saw very few people using the feature, only my girlfriend and that was because I showed her where it was and how it worked.
I think this kind of highlights the problem of modern Apple. They are very large and very spread out. In a perfect situation where Apple was still running like a well oiled machine then this issue of discoverability would have been dealt with in a .5 software update.
I’ll also point out that there is another key feature from last year’s WWDC that falls into this category. That of really not being complete and needing revision in order to properly thrive. That is the facial recognition and analyzing feature in Photos app. The lack of syncing between devices is a horrible omission and one that should have been added in a .5 update.
At this point we have to hope both are addressed in this years WWDC and not forgotten in favor of newer features which can be marketed.