At the moment the opening and closing of the off canvas preview isn't announced (see offcanvas.mp4). in comparison the announcement feeds module (announcements.mp4) and layout builder (layoutbuilder.mp4) the opening isn't announced either but it is communicated that there is a new web dialogue. the successful closing isnt communicated.
i am setting the issue postponed for now. @mikegifford contacted corbb from the national federation of the blind to get a direct feedback from a screenreader user. he is basically in line with the idea of announcing the opening and closing but is at the same time worried that it might be too much and be overwhelming. therefore his recommendation is to get direct feedback from a screenreader user and discuss the matter in a upcoming a11y office hour.
Postponed
1.0
Code
It affects the ability of people with disabilities or special needs (such as blindness or color-blindness) to use Drupal.
If we need to add annouce to the opening / close of an off-canvas dialog we can look into what we can do from a JS perspective to inject these behaviors on load.