To put it simply, in D&D 3.5, touch AC excludes armor bonuses when armor can't help against an attack, and flatfooted AC excludes your Dexterity bonus when you haven't acted yet in combat.
Mearls explicitly mentioned the design decision to not include Touch AC nor Flat-footed AC in the game during the playtest period. They are intentionally absent as a simplification which streamlines play.
Stoppable by armor? That's an attack. Not stoppable by armor? That usually requires a saving throw. #DnD
While not entirely accurate a portrayal of the spell list, it's substantially true. (In other words, there are exceptions.)
No. There is no mention of touch AC or flat-footed AC in D&D 5th Edition source books.
If you want to use those rules, you will have to find a homebrew system, or adapt them yourself.
While the concept does not exist, it is not as needed as in previous edition or Pathfinder.
Because of bounded accuracy: simpler math, less number explosions
Spellcasters now use their spellcasting ability to make their attack rolls, instead of Strength or Dexterity.
Less game math since gaining advantage will address most of these issues.
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