In some Flash games, specifically one where user-inputted text is displayed in a certain way, external .swf files that contain ActionScript code can be loaded and their code run. Other games may be able to use a loader to run code in a similar way.

Do you (pl.) think this form of ACE, which requires external files and a breach of copyright law, should be allowed? Obviously individual games have different rules, but I want to find out what speedrunners of other games think about this idea.

No, anything that requires external anything (hardware, programs, code files) should be, and usually is, banned.

I can write Lua scripts to do almost anything in NES, SNES, N64 games and then run them against the game. That doesn’t make it ACE, it makes it TAS.

ACE is executing your own code, written within the game, when the game is running. External code files aren’t so much ACE, they’re closer to TASing.