Currently, Power BI does not have a native built-in subtitle feature for visuals within the PowerBI client. But, it can be mimicked by making use of the "Title" property of visuals. You may enable title formatting to include title and subtitle, like with line breaks in the title text itself, for example. Another method is to use a text box below the visual to do manual subtitle positioning and resizing. While this does not directly compare with the native subtitle mechanism, it provides additional context to your visuals.