At this time, the standards for mobile accessibility are the same as for web sites—that is, the latest Web Content Accessibility Guidelines (WCAG), level AA. Although most WCAG guidelines apply to web pages viewed on both desktop computer and smaller mobile devices, a few accessibility issues are unique to mobile devices.

In WCAG 2.1, these guidelines were introduced specifically for mobile:

• 1.3.4 Orientation 
• 2.5.1 Pointer Gestures 
• 2.5.2 Pointer Cancellation 
• 2.5.4 Motion Actuation 

Also see Mobile sites and application testing.

Resources

Apple iOS

• Building accessible apps
• Apple Accessibility
• UIAccessibility

Android 

• Android Developer: Build more accessible apps
• Android Accessibility Help
• Android Accessibility Tutorial: Getting Started
• Android Accessibility: The Missing Manual
• Material Design: Accessibility

Helpful tools, guides and checklists

• BBC Mobile Accessibility Guidelines
• Rob Whittaker's Mobile A11y Resources

W3C documentaton 

• Mobile Accessibility: How WCAG 2.0 and Other W3C/WAI Guidelines Apply to Mobile (2015)
• Mobile Web Application Best Practices (2010)
• Mobile Web Best Practices 1.0 (2008)