Summary
Normally an above-average shooter from midrange and three, Williams struggled this season, and it caused his eFG% to drop to 46%, near the bottom of the league for his position, per Cleaning the Glass, and his lowest mark in a decade. Age may be catching up to Williams, as he'll be close to 35 when next season starts.

While Williams has plenty of foul-drawing tricks to keep him useful as he loses a step, that ability has allows waned when the games matter the most, as refs don't seem to give him the same calls he gets in the regular season.

At this point in his career, Williams is likely best served on a team where he is a luxury rather than someone who is needed to produce every night. If a team is deep enough that they can use Williams if he's hot but sit him if he's cold, they may show interest in Williams.
 
Cap Considerations
Williams will be an unrestricted free agent with a $15.2 million cap hold and full Bird Rights. Since the Hawks will likely be operating over the salary cap, they will retain Williams's Bird rights, and have no restrictions on re-signing him. However, the luxury tax could be a concern, as re-signing Williams could push them into the tax depending on what happens with their other free agents.

Therefore, other teams will have a legitimate shot at signing Williams away from Atlanta without having to make him a huge offer. Other teams will have multiple ways to sign Williams 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.