Yes, that is true for any mutable struct. Since the issue was about RefValue and why they are not exported, I thought they were mentioning issues specifically with it, not the general drawbacks of using mutable structs as fields of immutable structs.
Although it indeed causes memory fragmentation, there are still tons of sound use-cases for that where this fragmentation doesn’t matter.
And if the sole reason for not exporting Base.RefValue is to prevent people from using a mutable struct as a field in a struct, I don’t think that objective is being achieved by not exporting it. Even worse, now people are unknowingly using an abstract mutable struct as fields on structs.