That would be +1 on shield, yes. But think of how difficult it would be to perform. You would need to connect with a late-hit nair on the frame before you land, meaning you would essentially have to frame-perfectly drift sideways into someone's shield.I'm not going to pretend I'm positive, but since Link's late nair above has 9 frames of shieldstun and 4 of shield hitlag, if we managed to space the very late frames of nair [frames 33-39] so that we landed the very next frame, wouldn't nair be +1 on shield at perfect spacing because you'd have 4 frames of landing lag and 5 frames of shieldstun? I'm not saying this is particularly useful but it does leave me curious if there's some way to apply this if my math is correct...
That said, small amounts of +/- on shield with Link don't necessarily matter. Spot dodge is invincible starting on frame 2, so even if you land a move that's -1 like a strong L-cancelled nair, you can still buffer a spot dodge before Fox could shine you out of shield. This won't work every time obviously, but it's a neat little frame trap that can let you punish characters going for what seems like an easy shield grab.
And let's not forget platform-cancelled nairs. Strong nair is -1, but a frame-perfect platform drop nair takes just 5 frames to come out. This means that repeatedly platform-canceling nairs on somebody's shield is impossible to punish with a grab (plus I think it's possible to space outside of faster OoS options because Link's feet are huge). This would be like... hugely difficult to do. But Abate does it with Luigi, so I bet somebody could make the same thing work with Link.
(Edited to fix my backwards use of plusses and minuses.)