1) I've traditionally assumed that the intent of this deflection limit is to limit the curvature induced in a brittle ceiling material such that unsightly cracking would be precluded. In this sense, the limit being imposed is indirect in the sense that what you're really after is not the absolute deflection but, rather, the curvature associated with that absolute deflection. Presumably, the assumption of uniform loading is built into that.

2) The obvious difference between plaster and GWB is the jointing. However, I don't see that the jointing reliably improves things relative to the plaster case. If you've got an 8' length of GWB located at midspan, it's fasteners will force it into the same curvature there that the plaster would have been forced into.

3) 1+2 = I apply the L/240 to GWB.

The code could probably use a modernization in terms of the materials they list for this stuff.

I generally use L/240, but looks like L/120 is ok. I mean sheetrock is flexible AF after all.

From their last sentence, I would be nervous using larger than L/240.

It would've been nice if NWCB mentioned L/180!

Thanks for the replies, KootK and XR250!