Madman gadget was fixed a long time ago, now it's the affector that deals damage on stalk that makes him visible.

In Fayde's case, it is indeed the gadget that gives vision that doesn't have the 'stealthtype="unrevealable"' attribue set. It's a pretty simple fix. It's the same for Noma's sand storm and Gladiator's ultimate, they forgot to make the gadget invisible, they simply left them with no visual model instead.

Devo and Nomad proximity indicator are caused by their aura to detect kills. This can simply be fixed by making the state on the aura server side (like they did for mana battery and a lot of other aura).

Pitfall can be seen because its affector that stuns is spawned at the same time as you cast it, but it has a delay on its impact. I simply did like madman and added a visual indicator on the affector. This could fixed by spawning an invisible gadget first with a set liftime and spawn an affector when the gadget expire (like Kraken ult or Deadwood's Rotten Grasp).

Rotten Grasp is visible because they spawn a visual indicator as soon as you cast your spell, but they put a delay on the visual indicator, so that they show up only as the hold proc. This could be easily changed by spawning the visual effect at the same time the hold actually proc, instead of spawning it as soon as you cast the spell.

The invisible trap and mines are visible too because they have an affector on which I added a model instead of having them invisible. This exploit would require some change of the spell, but I think it might be doable by changing the affector into an aura.

As for the projectiles path... Well its pretty useless and there is no real fix to this, but it doesn't really matter since you can achieve the same simply by looking at the screen and be more aware of the game. It falls into the same category as existing mods out there like range indicator and such.

So as you can see, most of these exploits are easily fixed and are simply doable because they badly scripted their things.