I knew that Little Mac had partial invincibility on his legs during his grounded Forward-B (potentially for a while, before you decide to swing), but after experimenting in Training Mode, I've found that it actually has full body invincibility at the very beginning, and then it goes into the leg invincibility. Frames 1-4 are completely invulnerable.
As a "panic button," this has good synergy with Up-B, which is completely invulnerable on Frames 1-3 (and hits on Frame 3), and a cross-up Forward-B is actually pretty safe, since it pushes shielding opponents away from where you're moving. Even when it's punishable, it's likely not as punishable as a missed Up-B. Also of note: D-Tilt is -7 on shield (assuming they drop their shield instead of jumping out of it), and everyone's Dash Grab is Frame 8 or worse (and almost all of them only last for two frames), AND you cannot put up a shield during your initial dash, so if someone dashes forward to try and punish D-Tilt, this can serve as a counter to immediate attempts at punishing. Frame 1 Jab and Frame 2 spotdodge (for the many Frame 9 Dash Grabs) are generally the go-to against it, but those can be baited out and punished; this creates more of a mix-up with where Mac ends up post-move.
Relevant characters that absolutely cannot punish it without a powershield are Mega Man (well, he can jump-cancel throw a Metal Blade already in his hand for some damage) and Villager, and there are a number of characters who can only punish with a frame-perfect (or very close; you cannot buffer a roll out of shield hitlag, as an aside) roll backward into the appropriate move (usually a Dash Attack or Dash Grab), such as Mario, Luigi, Lucario, Ness, Pac-Man (without a Key, of course), and Wario; this seems rather hard to do in a vacuum, let alone on reaction in an actual match.