Untouchable is a bit overrated tbh, not every deck uses kill spells. But if a deck with kill spells sees a card with untouchable it just uses its kill spells on something else (the same applies with spells like stumble). Untouchable is much more effective when everything has it, or when you're attacking alone e.g. when using Azeal or Radariah respectively.
Untouchable doesn't make your card invincible to removal spells, it's still vulnerable to AOE removal, especially Mass Death. It just slightly limits your opponents options. But it doesn't really make you feel that much safer, especially on a card without flying, so it doesn't really increase your own options.
Untouchable actually reduces your options, especially with GI. GI is very focused on getting efficient trades, using aleta to buff cards so they can get a perfect 2 for 1 trade (e.g. using a 13/13 splitter bot taking out two opposing 10/10 splitter bots. This is an option that untouchable would remove.
Finally, splitter bot is a card that people avoid trying to kill unless they have to. In it's raw form it's just a 10/10 for 4 cost and doesn't get its full value until it dies. So people prefer to use their removal on other cards, e.g. on your 10/10 infected drone. So I'm not sure how much you're gaining by using Symbiote Prototype.