since smartthings hardware is no more, could someone recommend a door/windows contact sensor that natively works with Smartthings please.
Smartthings is no longer going to sell or support smartthings branded hardware, but the line is being turned over to partners. In Europe and in the United States, the partner is Aeotec. Initially, the devices are identical to the existing smartthings devices, just with a different logo. They are just starting to be sold.
So if you like the smartthings sensor, you can just buy the Aeotec version instead:
Otherwise, let us know what country you are in, as the device selection does vary.
Also, are you open to Z wave or do you only want Zigbee?