I work for a county. We currently only have disclaimers on public facing webmaps. We only use field maps for internal staff, so I give staff verbal disclaimers when I show them how to download field maps. We go over which datasets are view only and which are editable, where the data came from and what the goals are for data collection.
I show them how to edit and how they can break things on accident if they aren't careful etc.
Is your organization private or government? Are you going to grant access to each user or are you making your data publicly accessible so they can access it anytime without your knowledge?
As a work around you could create a polygon layer that covers your whole area that blacks out the map and has a pop up by default that they would have to read and close the pop up. Then they could go to the layer list to turn off the layer. Once they turned off the layer then it would stay off by default unless they turned it back on. So this doesn't really meet your criteria to force them to read it and force them to click on it each time they open the app.
