Today, plain text adoption often implies markdown for a richer experience. There are loads of markdown apps on the app stores, but the lesser-known org markup continues to fly under the radar. Quite possibly because of its Emacs origin (and many folks moving to other editors), but org is still plain text (and super powerful). We can all benefit from wider adoption beyond Emacs.
I built two org-powered apps for iOS myself:
https://plainorg.com
https://flathabits.com
There are other great ones out there:
https://beorg.app
https://braintool.org
https://logseq.com
https://organice.200ok.ch
https://orgro.org
http://orgzly.com
Shoutout to Karl Voit who's been driving org markup awareness outside of Emacs with Orgdown https://gitlab.com/publicvoit/orgdown. He's also discussed org markup's strengths at https://karl-voit.at/2017/09/23/orgmode-as-markup-only