Asked and answered in Post #10: site themes can hide It’s Off-Topic and It’s Inappropriate options from the prompt.
Personally I think - when there is an opportunity - a site admin should edit the relevant localization files (client.en.yml) and provide site-specific instructions, or maybe a link to a write-up on how to use the report post feature. I was myself confused as to the new process and had to ask early on.
Ideally, and this is more of a discussion for meta.discourse.org, the comment box would be optional for all selections. If users want to provide more details there is a place to do so, and it saves time for mods who don't have to PM to get necessary details. Users who don't need to add additional comments won't do so; moderators who don't need the additional comments to respond can ignore it - there's a character limit already to encourage conciseness. Seems like a win-win.
~Max