Canvas is for drawing shapes, primarily. It has some basic font support.
The browser can’t really do much with those shapes in terms of accessibility. Perhaps zooming or increasing contrast? The cases where it makes sense are few. Also, svg support has gotten better, and is easier to use, if you need simple animations embedded within a regular, otherwise accessible page.
Paths may be hit areas for UI interaction, though. I've no idea to how this translates to accessibility and if any of this can be exposed.
General advice: treat it like an image. If an image is good for the job, a canvas will be as well. Otherwise, if it is essential for navigation or getting the gist of that page, you may consider complementary elements or an all together different approach. However, if there is no sense in an alternative representation (like in the case of an arcade-style game), this type of content should be still allowed.
The browser can’t really do much with those shapes in terms of accessibility. Perhaps zooming or increasing contrast? The cases where it makes sense are few. Also, svg support has gotten better, and is easier to use, if you need simple animations embedded within a regular, otherwise accessible page.
Hope that helps.