SecurityTransparent is a bit of a vestige nowdays, but is still enforced on desktop runtime.
As such the presence of the attribute is a blocker for the new language/runtime features such as ref returns, readonly references, spans...

Use of readonly references, in particular, makes a lot of sense, since the collections are Immutable

NOTE: Nearly no other library in CoreFx uses SecurityTransparent. ( https://github.com/dotnet/corefx/search?utf8=%E2%9C%93&q=SecurityTransparent&type= )

It is surprising that Collections.Immutable still does.

I think the attribute needs to be removed and soon since it is already causing troubles.
(Example: change dotnet/corefx@fc4b1ad when upgrade to C#7.2 had to be paired with disabling certain optimizations - we are not even using new features yet)