SELECT
p.ProductID,
Relation = CASE x.n WHEN 1 THEN 'Linked' ELSE 'Not Linked' END
FROM @product p
OUTER APPLY (
SELECT TOP 1 n=1
FROM @ProductRelation r
WHERE p.ProductID IN (r.FirstProductID, r.SecondProductID)
) x
ORDER BY p.ProductID
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden