KaiOS is the most prominent alternative to either iOS or Android on mobile devices. Its CEO, Sebastien Codeville, has just given an interview outlining this platform's directions and goals for the immediate future. They include greater functionality, more form factors and new markets.
https://www.notebookcheck.net/KaiOS-to-expand-on-various-vital-levels-in-the-coming-year.423930.0.html
I'm really interested in KaiOS, especially if a QWERTY phone with a good camera appears.
Also, I find it rather baffling that more than 1 GB of RAM is considered necessary for multitasking for Android and iOS. Just goes to show how extremely bloated they are. Multitasking worked perfectly on my old Amiga 4000 that had 8 MB of RAM, and I did the same things I do now on a phone: email, annotations, light browsing, image viewing, listening to music, etc. I know the web has become extremely bloated the last years, but a lightweight Linux distribution and 512 MB RAM gives a great browsing and multitasking experience even today...