I suggest you reach out to Athom with your request, via https://homey.app/support/. The sustained value-functionality is not app specific, but a Homey wide functionality. Currently limited to numeric capabilities only.
There is an alternative though: the Device Capabilities app provides a flowcard to check for sustained values. It also works with yes/no and text values, in addition to numeric ones.
Also check this post (in Dutch) for screenshot and more info:
