It contains the first chumra elevated to the status of halacha without proper explanation. Chumras as safeguards to laws is, of course, part of Mesorah.
Asu seyag laTorah is one of the earliest statements attributed to the Anshei keneses hagedola. It is also one of the first to be documented as being attacked by the anti-chumra crowd. Here is noted scholar Lawrence Schiffman quoting the Sadducee fragments of the Dead Sea Scrolls: