Immutable function returns over collections

[markfinal]

This is for the next major/minor upgrade.
But functions that return lists, if they are really just to iterate over, and not mutate, should be as fast as possible, and indicate immutability.

For example, https://stackoverflow.com/questions/24880268/ienumerable-vs-ireadonlycollection-vs-readonlycollection-for-exposing-a-list-mem/24881237.

[markfinal]

Is this still valid? A lot of thing changed to IEnumerable<> in 2.0