The secondary-selection is used when the cursor is in some focussed Recipient window, in order to grab some text (or whatever) from some Donor window (possibly the same window as the Recipient) and have that text pasted at the Recipient’s insertion-point (overwriting any primary-selection in the Recipient).
Why do you think it will be dropped? The discussion around it has been about disabling it by default and adding an option in the DE settings that would also serve explain the feature.
It’s already implemented in every Wayland DE and many users are used to it, it doesnt really make sense to remove it.