Your save data is permanently tied to the platform account it was created on. Cross-Save does not move or transfer your save data in any way, it simply lets you load that save data on other platforms. If you choose to disable Cross-Save, your Xbox save data will only be available when playing on Xbox and you'll only have access to your Steam save data when playing on Steam. If you didn't previously create any characters or play on Steam prior to enabling Cross-Save, then you'll be starting fresh on Steam.