From #88093 (comment):

We should revisit this.

runtime/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableExtensions.Minimal.cs

Lines 93 to 96 in e5ddc6c

	/// The reason we don't copy anything other than for well-known types is that a malicious interface
	/// implementation of <see cref="ICollection{T}"/> could hold on to the array when its <see cref="ICollection{T}.CopyTo"/>
	/// method is called. If the array it holds onto underlies an <see cref="ImmutableArray{T}"/>, it could violate
	/// immutability by modifying the array.

That logic, while well-meaning, isn't worth making the implementations slower. If code wants to get at the underlying array of an ImmutableArray, there are easier ways, including a now fully supported public API.

The comment that triggered the conversation #88093 (comment):

The custom ToArray implementation is slower, because it uses CopyTo only for collections that implement IList<T>, while both KeyCollection and ValueCollection don't. They do implement ICollection<T> which is used by LINQ.

cc @stephentoub