There are feats that are lower than that, as seen in First Contact and a few other times.

There are also higher like, as seen in a recent ST: Discovery episode, and a few other times.

There are character statements (like in TOS: Taste of Armageddon for instance) that can be construed to be in the low megaton range, but they are vague enough they also could easily be higher or lower.

But I can't think of any that would specifically support low megaton range for their torpedoes like what is given in the TNG Tech Manual. I guess maybe the impulse engine self destruct from TOS: Doomsday Machine? But even then that has some problems with it. It also isn't a torpedo feat.

If I am forgetting anything I would be interested in hearing about it.