In ColumnTransformer, #14048 raises the question that whether the output feature name should be prefixed when the transformer is set to passthrough, or the feature is in the output as a part of remainder=passthrough'`.

The PR is mostly finished and awaits a decision on this question, hence opening this issue to discuss the alternatives.

We have the following two options for each case:

1. prefix the output feature to keep it consistent with the case where there is indeed a transformation happening on the column
2. do not prefix the output feature name since there has been no transformation done on it.

We don't have to choose the same answer for the two cases (transformer is passthrough or it's a part of the remainder).

ping @jorisvandenbossche @jnothman @amueller