The most updated version of HEC-GeoHMS (10.5.0.7) was published on Aug 30, 2017. I highly doubt there will be future versions developed for HEC-GeoHMS. Whoever wants to use it would have to stick with ArcGIS 10.5 assuming all the tools work as intended.
There is a similar question on Research Gate where posters had to downgrade to older versions
I am able to run Preprocessing, Project Setup, and Basin Pro When I run characteristics or parameter...
If you decide to downgrade, make sure to use "Save a copy" option so that you can open the MXD files with the older versions of ArcGIS
Will ArcGIS 10.6 mxd open in 10.5? - Geographic Information Systems Stack Exchange
Otherwise you'll have to use the MXD Doctor utility
Using the MXD Doctor utility—Help | ArcGIS Desktop