Looking on the Lichess database I can see that 6. f3 is the sixth most common response by masters with a 6% rate. Now I've downloaded the latest version of Lc0 (0.31.2) with the heaviest weights and ran position startpos moves e2e4 c7c5 g1f3 d7d6 d2d4 c5d4 f3d4 g8f6 b1c3 a7a6 with multipv=4. And the strongest opening player in the world by far (Lc0) has said that 6. f3 is 10 centipawns better than the second best move, Be3 (the mainline for masters). While Masters and human-based opening theory generally understand the opening very well, I think that Masters should not be dismissive of engines as they are the best players in the world. I think a lot of people don't like engines because of Stockfish, which is obviously flawed in opening theory but only because it is based on human principles. Lc0 has the 0 for a reason and players should not be dismissive as its strategy is off the charts (the best in the world). So yes I believe that 6. f3 is a heavily underrated opening.
Sicilian Najdorf 6. f3?
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It's just a game. White can answer 6...Qb6 with either 7.g4 or 7.Nb3. Nobody is better, they just... play chess.
If Black does not want to study a second line for no reason, then 6...e5 is perfectly acceptable- the play will almost surely transpose to the 6.Ne3 e5 7.Nb3 sharp variation of the English Attack.