Like boots protect you from acidic slime molds...
I'd rather it start eating the BOOT before it gets to my precious skin, as opposed to letting it have the flesh as an appetizer before it starts in on the bone...
Ah, I can't even imagine a DM that would warn you about a jelly eating your boots before it gets to your skin. You pass the perception roll (or whatever it is), you're ok. You fail it, say goodbye to your limbs no matter what you wear.
Warn them? No. Hint about their presence though descriptive atmosphere setttings? Yes.
And boots help with that in... what way?
They at least provide some protection from the acid. At least some enchanted leggings? Or some medieval/fantasy equivalent of climbing or running shoes for those rather risky dungeon terrains. Who knows what the sharp, pointy objects are down there? 
No they don't. They only provide armor class, which is never taken into account with damage like acid. You need a saving throw for that.
Plus, helloo? Wizard? Levitation? Y'know?
Plus, helloo? Wizard? Levitation? Y'know?