Per proposal spec,

PipelineExpression[In, Yield, Await]:
    LogicalORExpression[?In, ?Yield, ?Await]
    [~Await]PipelineExpression[?In, ?Yield, ?Await] |> LogicalORExpression[?In, ?Yield, ?Await]
    [+Await]PipelineExpression[?In, ?Yield, ?Await] |> [lookahead ∉ { await }] LogicalORExpression[?In, ?Yield, ?Await]
    [+Await]PipelineExpression[?In, ?Yield, ?Await] |> await

The solo await expression await is only allowed when it is the only production within the pipeline body, there is also a specific await lookahead before forming the LogicalORExpression. Previously Babel allowed solo await within the pipeline body even if the await expression might not be the only production. For example, the following cases were accepted by the current Babel parser but forbidden by the proposal spec:

async function test () {
  x |> async () => await;
  x |> (await);
  x |> await || true
}

The new approach only allows forming AwaitExpression when it see await after the pipeline operator.

We also banned binary operator after solo await as the LogicalORExpression can not cover PipelineExpression, users should parenthesize the complete solo-await pipeline if it is used with a binary operator.