Some fighters have jab cancel KO confirms, such as Dr. Mario, Isabelle and Snake.

Almost every single jab in this game sends at a trajectory of 0 with minimal scaling, however a lot of them actually gain you frame advantage when your opponent is around 100%.

The issue is that most fighters don't have fast enough ground attacks to truly make use of it; Palutena for example has around a 3 frame advantage at 130%. You can get a grab if you predict shield, roll or jump but it will lose to a fast jab or buffered spotdodge.

It also depends on the placement of the jab; Snake has to hit you point blank to get the best knockback which gives around a 5 frame advantage at 140%, enough to buffer into crouch+c-stick tilt forward to combo into forward tilt. It's difficult, but doable. Isabelle's jab to forward tilt is also true starting around 140%.

Dr. Mario might be the easiest out of all fighters, he starts to get frame advantage around 90% and by 120% Jump Punch becomes true.

Bowser gets frame advantage starting around 100% but it's a mix-up until around 180% where Flying Slam becomes true.

None of this is relying on Training Mode's combo counter as it is inaccurate. So yeah, hit the lab to see what kind of jab cancels your fighters are capable of.