It wouldn't be surprising if Stills is on the trading block with a trade pending the outcome other competitive positional battles around the league. Kenny's skills probably makes him a fit for lots of teams and his well-established routes should make it easy for most QBs to work him into their offense. On the other hand, the positions that Dolphins may seek may require the 3rd preseason game to resolve such competition.
Looking back, the emergence of Preston Williams likely changed their plans for the WR corps. The signing of Allen Hurns could indicate that the team wants a competitive battle with Isaiah Ford & Trent Irwin for a 3rd WR role, where they likely desire a more move-the-chains possession slot receiver. Meanwhile, DeVante Parker is already benefiting as he no longer covered by Xavier Howard (i.e., the best corner) on all critical plays and will likely face more 'winnable' coverage during the season.
Williams leap-frogging the depth chart makes Stills expendable. The offense is doesn't require more than 3 WR sets and will frequently use less than three. Wilson, Grant, and O'Leary have lined up in the backfield, which allows the team to still run the routes to spread a defense, while making the it think about the potential for run/pass, containment, and trick play.
I have no idea what Stills may bring in a trade, but the team missed the window where veterans with high contracts were traded, such as Kelechi Osemele.