Rclone has been unblocked!
I got this reply from Microsoft:
I've updated my systems and it now works as it should.
Please all update our Defender signatures, disable the exceptions and report back if it doesn't work for you with a signature version equal to or greater than 1.347.597.
EDIT: It seems that Smartscreen has not yet been updated. If you download a new copy of rclone, you may still see a red warning screen.