Is simply changing a local state variable in a composable considered a side effect?

Yes

But it's not mentioned under side effects docs

You are right. I think the compose docs on side effects are mostly talking on the effect api

@Composable 
fun dummy() {
  var foo = 1
  TextField("Hello")
  foo = 2
  if (foo == 2) {
    TextField("2")
}

You should not expect seeing both TextField, and should use

mutableState

, which tells Compose to recompose (update) the execution. AFAIK ripple animation is a class containing mutableStates via a delegate, so updating these

local

variables are fine