As soon as you enter a PVP battle you will automatically be “entered” into the season so there isn’t a way to opt out unless you don’t battle at all. Anyone who had trophies over 5000 were reset in preparation for the season and they will be reset at the beginning of each season from now on. The rewards this time around are based on trophy count instead of rank and it is based off of your highest achieved trophy count, not where you are when it ends. When you click on the Leaderboard from the battle menu, you will see your position, your highest achieved trophy count and your current trophy count.