iPhone x compatibility with apple watch is compatible?

It's the currently-loaded OS which matters, not the original one. The only units which will be on a compatible version for certain are the Series 3 and earlier ones; later models will only work if they've been in storage for a while or were previously owned but not updated to 10.

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Any unit on an OS below 10.

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iPhone Xs or newer.

The X is one year too old