Is it true that TCR bans the use of the term “investigative journalism” ?
Yes.  It’s redundant. A journalist’s job is to investigate.
The term is a product of a new phenomenon born out of digital communication or journalism by email, Tweet, etc. which is intended to distinguish actual reporting from what most ‘reporting’ is now, in the post-Internet age. Woodward and Bernstein were not “investigative journalists.” Bob Woodward was a cub reporter on the metro desk assigned to cover a local burglary, Watergate.  Woodward would be the first to say – and does say – that journalism is about showing up and outworking every person around you. All journalism demands showing up and being a surrogate for your readers.