Slade featured for England at the last two tournaments and has been a regular since making his debut in 2015, but was dropped by former coach Eddie Jones late last year.
He has not done enough to convince Borthwick that his versatility, which has also seen him previously also play flyhalf, fullback and wing for England, is enough of an asset.
Number eight Alex Dombrandt, flanker Tom Pearson, hooker Jamie Blamire, centre Guy Porter and wing Joe Cokanasiga also miss out.
Squad:
Backs: Henry Arundell (Racing 92), Danny Care (Harlequins), Elliot Daly, Owen Farrell (both Saracens), George Ford (Sale Sharks), Ollie Lawrence (Bath), Max Malins (Bristol Bears), Joe Marchant (Stade Francais), Marcus Smith (Harlequins), Freddie Steward (Leicester Tigers), Manu Tuilagi (Sale Sharks), Jack van Poortvliet (Leicester Tigers), Anthony Watson (unattached), Ben Youngs (Leicester Tigers)
Forwards: Ollie Chessum, Dan Cole (both Leicester Tigers), Tom Curry (Sale Sharks), Theo Dan, Ben Earl, Jamie George (all Saracens), Ellis Genge (Bristol Bears), Maro Itoje (Saracens), Courtney Lawes, Lewis Ludlam (both Northampton Saints), Joe Marler (Harlequins), George Martin (Leicester Tigers), David Ribbans (Toulon), Bevan Rodd (Sale Sharks), Kyle Sinckler (Bristol Bears), Will Stuart (Bath), Billy Vunipola (Saracens), Jack Walker (Harlequins), Jack Willis (Toulouse).
