Routers and WiFi devices have to undergo [certification][1] to have the "WiFi" branding. Any router that does not have a 2.4 Ghz radio would be lacking support for [IEEE 802.11 b/g/n][2] and surely would fail certification.
 
That means there is *no such thing* as a "WiFi" router with only a 5 Ghz radio. You can disable the 2.4 Ghz radio in many routers though.

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 Ghz devices since the certification is not there. Some day the WiFi Alliance may decide to deprecate the 2.4 Ghz standards and this answer may no longer be correct.

  [1]: https://www.wi-fi.org/certification
  [2]: https://en.wikipedia.org/wiki/IEEE_802.11