My Windows 10 Insider test system (Latitude 7350) just updated to the mainstream build Windows 10 Version 1803 (March 2018) also known as Build 17133, Redstone 4 (RS4).
This is a mainstream build, at current the Windows 10 Media Creation Tool is not updated to the latest version however I expect in to be updated within the next 2 weeks.
The watermark is gone:
msinfo32 shows no expiry date (meaning it is not an Insider build):
The build is also shown in System: