The title is slightly misleading. Inline assembly code in asm statements has no effect on the stack.

If you have your own .S files, and those do not contain the magic handshake, then the stack will stay executable, and this is indeed a programmer trap.

The reason for this is that historically the stack was always executable, so it could not be make non-executable by default, it had to be opt in. In fact, gcc will generate code that requires the stack to be executable if you use a certain gcc extension (related to nested functions).

The other way to opt-in into non-executable stack is passing --noexecstack to as(1) (or when via gcc, use -Wa,--noexecstack)

This ought to be better documented, but it's not. The rationaly is probably that you should not be writing assembly code if you don't know this kind of detail? I can only guess about this.