Summary
In the '16-'17 season, Johnson was one of the most improved players in the league. He went from a lower-end bench player on the Raptors to an on-ball playmaker for a Heat team that finished the season on a 30-11 run after starting the season 11-30. He turned that into a 4-year, $60 million contract that summer, but was never able to replicate his performance.

Johnson has shown flashes that he can still contribute, but it hasn't been consistent enough that I would trust him as a rotational piece anymore. He has decent enough touch from midrange and outside, but his low three-point percentage (26%) over the last two seasons is a bit concerning.

However, despite shooting poorly from pretty much everywhere on the court, Johnson gave the Nets some solid minutes this season. He's a decent playmaker, and can make just enough plays off the dribble to be a threat with the ball in his hands. As the screener in pick-and-roll, he has the ability to make plays in 4-on-3 situations, and can operate as a pick-and-pop threat, despite struggling from outside.

He's a bit slow-footed on the perimeter defensively trying to stay in front of wings. He can operate as a small-ball center, but doesn't provide any rim protection and doesn't rebound well. At age 35, Johnson is probably just your classic veteran minimum type, but I think he's shown enough to stick around for a little while longer.
 
Cap Considerations
Johnson was waived by the Nets near the end of the '21-'22 season, and did not finish the season on an NBA roster. 

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, Room MLE, or Bi-Annual Exception to sign him. It's also likely that Johnson only gets minimum offers.