Summary
After signing a surprisingly-lucrative two-year $7.5 million contract in 2019, Johnson almost immediately fell out of favor in Toronto. Coach Nick Nurse publicly criticized the defensive abilities of the newcomers, which included Johnson. While he got some scattered minutes throughout the regular season, he did not see any real time in the playoffs in 2020, and only played sparingly for the Raptors this past season, even when they've suffered numerous injuries. Johnson's offensive game has not developed much since entering the NBA.

Despite being considered a wing when he entered the NBA in 2015, his best position is probably at power forward (or maybe even a small-ball center) now because of his limitations on offense. Johnson cannot create offense for himself, and hasn't shown to be much of a playmaker either. But take Johnson away from the perimeter and into the high post or dunker spot, and maybe Johnson can make some simple reads and thrive at that position. Johnson is strong enough and athletic enough to guard multiple positions, but has not developed into a good enough defender to offset his offensive deficiencies.
 
Cap Considerations
Johnson will be an unrestricted free agent with a $4.9 million cap hold and Early Bird rights. Using his Early Bird rights, the Raptors can offer Johnson a starting salary of 5% above the average salary (around $10 million), which should be more than enough. In the unlikely scenario that it's not, the Raptors could also use cap space to re-sign Johnson.
 
Other teams will have multiple ways to sign Johnson as even teams without cap space could possibly use any of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE or Bi-Annual Exception to sign him. There's also a chance that Johnson ends up settling for the minimum.