I didn't realise that OP3 kept data on podcast apps and their overall share of downloads. But it does. And so, now does the Podcast Business Journal. I should seek out more stats for this. https://podcastbusinessjournal.com/data/podcast-apps/
I've also added some of the data from the Podcast Index's stats here, under the guise of "new podcast features" - https://podcastbusinessjournal.com/data/podcast-features/ - which suggests that shortly we'll hit 2.5mn episodes with creator-supplied transcripts.