So this article was from 2011, saying that he was. Granted, it was dated late May, so one could argue that's more of a "voluntary OTA" type of workout then a side workout.
https://bleacherreport.com/articles...n-informal-workouts-but-is-it-enough-for-some
This one was from late March, 2014:
https://www.espn.com/blog/baltimore...6/joe-flacco-planning-workouts-with-receivers
Technically it says "was planning" so I can't prove it happened, but I'm assuming it did.
I would tend to agree with most that through his rookie contract there's very little evidence he did any of this. 2014 would seem to be the first time he did. Naturally he gets hurt during the 2015 season, so he would have likely skipped the pre-2016 workout. There were some questions about his availability for pre-2017 season workouts. He had a back disk hernia that offseason, where I recall he did not play in the preseason at all. No real idea how long that injury lasted, but if it lasted several months, that would probably justify a lack of workouts prior to 2017 also.
I'd give him a "pass" for both offseasons as a matter of just staying healthy. But agree there's really no excuse for the first call it 5-6 years of his career.