Archaeologists: “Uhhhh, there’s still a lot of debate about how effective leather armor really could have been on a battlefield. Alas, we shall never know.”
Punks: “Hey, fresh cut, the boneheads carry knives sometimes so make sure and lift a good leather jacket. It’ll save your life.”
Layers layers layers! Slashes won’t do shit even to most t shirts but a stab will ignore the shit outa your leathers. Layers will keep the blade from getting as deep as it otherwise would and gives more for it to snag on if it serrated.
Armour has always been about layers.
Example 1200s minor noble: linen shirt, gambeson (layered and quilted linen with wool insulation), chain mail, surcoat, arming cap, helmet, coif, bigger helmet.
Another example Alexander era Macedonian hoplite: linen tunic, greaves, 1" of tightly pressed and laminated linen, helmet (probably with some sort of arming cap/padding inside), big ass shield.
Layers save lives.
Leather armor is just a more expensive gamerson. Tough leather jackets are basically comparable to the later buff coats that replaced the gamberson in the age of gunpowder
Modern leather jackets really are made of thick enough leather to protect you from shit all. Having a bunch of studs & spikes can help but only marginally so. Heavy duty layers are the way to go.
Laminating layers of fabric on top of one another or quilting with wool stuffing in between layers are tried and true armour types and can be worn pretty inconspicuously under a sweater or jacket or whatever.
If you’re going to be at an event with the real possibility of getting knifed spend the extra time armouring yourself properly.