There wont be a work stoppage in 2021. Simply wont be.
The NFL has inexplicably developed this reputation as a dysfunctional organization that cant keep its shit together but last I checked they bring in billions and billions every year and are gaining global popularity year-by-year. It was so trendy in 2015-2017 to say "Oh, I'm quitting the NFL! Its so bad!" but the numbers of people who love the league and follow its every move keep going up. The players and owners will not upset the gravy train with a work stoppage. The kneeling, concussions, player misconduct etc., none of it even dented the league's popularity. The only existential issue facing the NFL now is the horrendous state of officiating and everything seems to point to a hammer coming down this off-season in regards to that.
Maybe the NBA or NHL could suffer another stoppage but not the NFL. I expect there to be some pretty major changes, like the franchise tag either going away all together or becoming more like the transition tag, but nothing that will shut the league down. Definitely there will be no guaranteed contracts and definitely the salary cap will continue to exist. May get another uncapped year though.