The 49ers are not picking up the contract option for veteran defensive tackle Earl Mitchell, a source tells Ian Rapoport of NFL.com (on Twitter). Mitchell provided the Niners with a solid locker room presence, but they’ll have to find that leadership elsewhere. 
The Niners used D.J. Jones as their nose tackle down the stretch of the season so it’s likely that Jones will continue to be the team’s answer in the middle, unless they replace him with someone better between now and September. Mitchell leaves the Bay Area two years into a four-year deal.
At the time of signing, Mitchell’s contract was reported to be a straight-up four-year deal with no option years. Instead, the deal was believed to be a four-year, $16MM pact with a $4.45MM cap figure for the 2019 season. Releasing Mitchell under those terms would call for a $1.5MM dead money charge with $2.95MM in cap savings. Regardless of whether there is an option or not, the outcome is likely the same.
Mitchell finished out the 2018 season with 28 tackles across 14 games (12 starts).
Our defense sucked again last year so cutting him is no big deal. Need to do some work on that defense big time!
Naturally no details could be provided as to how Earl provided solid locker room leadership. We can assume that part was a bit of creative fiction since no team is going to eat dead cap money to rid themselves of a leadership asset.