I think it will be good to put this into its own type:

struct LocalDraft
{
    std::string text;
    std::string relType;
    std::string relatedEventId;
};

Then, instead of using a simple string of oldDraft, we can store all draft versions in a immer::flex_vector<LocalDraft>.

When the user clicks Reply or Edit, push a LocalDraft into the vector. When the user clicks Close and confirms, pop the last object from the vector. The current draft can be obtained by the last element from the vector (or a default-initialized LocalDraft when the vector is empty).

SaveLocalDraftAction will be changed to only modify the current draft.