The conformance was removed as part of migrating these types into the standard library, because it doesn’t really make sense semantically. As a workaround, you can use the .indices property, which conforms to collection both before and after 5.0:

v.indices.map { v[$0] }

The dropping of Collection is not a bug, but we may want to consider adding Sequence conformance at some future point.

An alternative to the Sequence conformance might be:

extension SIMD {
  /// A random-access collection of scalars.
  public var elements: LazyMapCollection<Range<Int>, Scalar> {
    indices.lazy.map { self[$0] }
  }
}