KMC and whipperman have a special Link that uses a Key hole lock to secure it in use then when you squeeze the rollers together it will Open
Now derailleur chains are made with such design tolerances that you remove a few links to shorten it, save that section .
because if you break the chain You have to remove the damaged section making it even shorter , then with the saved length and a couple More quick links you restore the length.
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side plate holes have to stretch to push the riveted pin head out the hole will always be loose ever after . so pushing the pin back in expect it to break ther again because it is loose .
and to fit more "speeds" the chains got thinner and the riveted pin is flush with the widest link of the chain so very little remains retaining it in a Loose hole.