They have no plans for superman sequels, Cavill took the witcher gig which will probably last 5-8 seasons (So he'll be tied up until like 2025) depending on how well it does.
This is either a negotiating tactic to force WB's hands into moving ahead with MOS2/a new deal so he can figure out the long term issues of Witcher/superman or he's actually leaving after his contract is done (Still has 1 movie left, supposedly going to be in Shazam as a cameo)
Who knows at this point, WB could cave to the backlash and sign him to a new deal or they'll just double down and stay silent because their movie slate is already filled out until 2020.