Last weekend I spent some time creating very basic tabular reports in SSRS 2016 to see how they handle accessibility, WCAG and Section 508 issues. It looks like things are largely unchanged since the first SQL 2008 Service Pack which introduced the AccessibleTablix property to the various render options (See this prior post). Now you will want to add the AccessibleTablix item to your HTML4, HTML5 and MHTML renderers. You will get tags linking the detail cells to the header but you will still be lacking a few of the items needed to pass a WCAG audit with flying colors (e.g. TH tags in the header row).
In my testing on the Chrome browser, the HTML 5 renderer seems to be the default used by SSRS. I assume the HTML 4 renderer still exists purely for backwards compatibility.
If you are wondering why Microsoft has not addressed accessibility better, it is probably a simple matter of priorities. When I ask people from other parts of the country (outside the DC region) about Section 508 they mostly have never heard of it. It’s similar with some of the security stuff we all have to deal with (e.g. FISMA). Unless and until it becomes a financial problem for Microsoft (i.e. big sales are being lost because of this), we will not likely see this change. Accessibility is of great importance to anyone working with the US Federal or EU governments (including Microsoft), so I hope future versions will include improvements.