Currently, it is possible to add alt text to images scheduled to Twitter through Buffer.
To add alt text to your Twitter image, click on the image you uploaded within your Buffer composer. This will open the image in a larger view and there will be a text box at the bottom where you can add your alternative text description, up to 420 characters.
Once you're done, click "Save".
While Facebook and LinkedIn do support alt text for third party apps through their API, it is not something Buffer supports at this time. We are committed to improving in the area of accessibility, and you can find our full Accessibility Statement here.