ELH p.21 says the class gets a feat every 3 levels after 20th, but the table on the next page shows bonus feats at 13th/16th/19th. It's not hard to imagine what's probably correct, but has this discrepancy been officially clarified anywhere?
It is a discrepancy and was not corrected.
Per the most recent errata (dated 2/16/2006) the probable typo has not been corrected. Per the general priority guidelines of the errata:
When the text within a product contradicts itself, our general policy is that the primary source (actual rules text) is correct and any secondary reference (such as in a table or a monster’s statistics block) is incorrect. Exceptions to the rule will be called out specifically.
and the lack of an explicit exception for the Shadowdancer, the table is incorrect and bonus feats are not available until after 20th level Shadowdancer.
In all likelihood, this is an uncaught typo and the table is correct. But, as currently written, the text is correct and the table is wrong.