Routers and WiFi devices have to undergo certification to have the "WiFi" branding. Any router that does not have a 2.4 Ghz radio would not be lacking support forbackwards compatible with IEEE 802.11 b/g/n and surely would fail certificationmay not qualify for the "WiFi" branding.
That means there is no such thing asI have never seen a "WiFi" router with only a 5 Ghz radio. YouBut you can disable the 2.4 Ghz radio in many routers though. It is possible that a router with only 802.11ac support can exist, but it may be hard to find a product like that using the "WiFi" brand as a search term.
For now, I would not recommend seeking out a product that explicitly avoids having a 2.4 Ghz radio. It wouldn't be guaranteed to work with all 5 Ghzother WiFi devices since the certification is not thereunless it was WiFi certified too. Some day the WiFi Alliance may decide to deprecate the 2.4 Ghz standards and this answer may no longer be correct.