[2021] What’s the full list of items you get in r6 siege when you preorder r6 extraction?

Technically there are no SIEGE rewards for pre-ordering.

You get the Orbital Decay bundle in EXTRACTION for pre-ordering.

For launching both SIEGE and EXTRACTION on the same Ubisoft account you will get the United Front bundle which contains rewards for both games. Plus you will unlock any of the 18 launch Operators available in Extraction on Siege if you don’t have them unlocked already.