Overall, Snapchat has limited accessibility features and other social media venues are encouraged to be used instead. If you must use Snapchat however there are things you can do to promote accessibility in your posts. Make sure your Snap Stories make sense from beginning to end. Storyboard your snaps to make sure the story is coherent.
The only way to make your snap story videos completely accessible is to upload them to YouTube and caption them there.
All your friends on Snapchat will benefit from snap photos and videos that have good lighting.
For everyday chats, use the largest possible text size and ensure that the text is legible against your background image.