Inputs can be added and removed if Inputs Modifiable is set, so I don’t think it’s a good idea to have anything that relies on the ordering of inputs to be consistent. Having any field that includes a list of indexes could become messed up by an unaware constructor that adds an input in the wrong place.
I remember discussing this with @josibake on the call, but don’t fully remember the conclusion. I think it was something like it wasn’t necessary to make any special consideration for ANYONECANPAY if we require that silent payments outputs can only be added if there are no inputs yet, or inputs with no signatures, or Inputs Modifiable is not set.
As long as an input uses SIGHASH_ALL, I don’t think an input with ANYONECANPAY is actually an issue.