In plain English
FAR 52.247-49, Destination Unknown, is a contract clause prescribed at FAR 47.305-5(b)(2), most recently dated Apr 1984. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 47.305-5(b)(2), insert the following provision in solicitations when destinations are tentative and only for the purpose of evaluating offers:
Prescribing reference: FAR 47.305-5(b)(2).
The official text, verbatim
As prescribed in 47.305-5(b)(2), insert the following provision in solicitations when destinations are tentative and only for the purpose of evaluating offers:
Destination Unknown (Apr 1984)
For the purpose of evaluating offers and for no other purpose, the final destination(s) for the supplies will be considered to be as follows: __________________________
(End of clause)