I think there are a lot more people checking the statistics for SR and the players take better note of what changes are made to the "main" game.
Showing the ARAM specific changes could negativly impact the choices of those players who very rarely play ARAM and have only little understanding of the ARAM specific strategies and playstyles. However people who play ARAM a little more but don't keep the ARAM changes in mind can benefit from that.
Getting more direct now: Assuming every champion had a 50% win rate due to some changes to damage dealt or similar. Let's say a Sona main would decide to stick to the champ in ARAM and then get into game. This player would be confused because the champion deals significantly less damage and takes significantly more damage than usual. Therefore I believe it would still be important.
Having it the other way around would be quite a help for people who don't regularly read the patch notes or returning players. I think it could be useful.
To be more specific about the flag: I imagine a little icon next to the reroll/lock in button that shows the information when clicked on or hovered.
Thank you for your response and input. I never thought about turning this idea the over way around.