Summary
Despite showing brief flashes here and there throughout his career, Parker has largely been a bust compared to the hype he had coming out of high school and when he was drafted #2 overall in 2014.

On defense, Parker's lack of lateral quickness makes it difficult for him to defend wings, and he hasn't developed the positional awareness that other athletically-declined players have used to survive on defense. On offense, Parker hasn't become a threat from outside, and his isolation game from the elbow is outdated in today's NBA, especially for someone that isn't elite at that skill.

Parker also suffers from being a bit of a tweener. He's probably best served to be a small-ball center in today's NBA, but putting Parker as your anchor on defense is suicide. Parker has hardly played for the Kings this season, and while he had some okay games for the Celtics, it's likely he's only looking at minimum offers this summer.
 
Cap Considerations
Parker's $2.3 million salary for 2021-2022 is non-guaranteed, and if he is waived he will become an unrestricted free agent and the Celtics will have no form of Bird rights on him. If he is waived, any team could claim him off waivers since he's on a minimum contract.

If Parker clears waivers and becomes a free agent, other teams will have multiple ways to sign him as even teams without cap space could possibly use either of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE to sign him. However, I don't expect any offers for Parker to be above the minimum.