Unless the rerouting means a missed port or delayed time into a port, there is really no need for everyone on the ship to be informed. Anything of import would be stated over and over and crew would be made aware to convey to you AND it would be repeated at the evening shows at the very least. If it's not repeated over and over, it's not really important to YOUR cruise.
Twice I have been on a ship (Celebrity both times) when there was something like that. The first time we were going to miss Costa Maya due to unrest there and Captain Kate announced it multiple times during the day prior, at the shows, and paper notices were put in our cabins. The second was on Celebrity Summit and on the way back from Bermuda the captain announced multiple times that due to a large cargo ship needing the full channel to exit Bayonne, we were going to be delayed arriving back into port. Again, that was repeated multiple times during the day, was announced before the shows, and a paper announcement was put in our cabins.
Based on the route shown on the cruise globe site for a few of my other cruises we likely had a diversion for some reason, but since it was none of our business as it didn't affect the majority of the passengers no announcement was made.