Let's say you have an existing enum in your library API:

enum SpanMode
{
    depth,
    breadth,
    shallow,
}

At some point you may decide you want to rename some of the members, but want to avoid code breakage for at least some time. You could introduce additional members which have the same values as the old ones:

enum SpanMode
{
    deep,  // new
    wide,  // new 
    shallow,

    depth = deep,  // old
    breadth = wide,  // old
}

This will work with switches, final switches, to!string, etc (although code that depends on EnumMembers!E.length could potentially break depending on what it does).

Don't mind the naming choice (it's just an example), but it would be useful to allow a deprecated statement in there, ala:

enum SpanMode
{
    deep,
    wide,
    shallow,

    deprecated("Please use SpanMode.deep")
    depth = deep,

    deprecated("Please use SpanMode.wide")
    breadth = wide,
}