Selection was introduced at the same time as “space bar cursor”, which is with the introduction of 3D Touch in the iPhone 6s.

Actually, it wasn’t even a “space bar cursor”, as the way of triggering it was with force pressing and not by holding the space bar. It works much better with 3D Touch, as you can move the cursor around and press harder without lifting to start selecting, then release a bit to stop. My iPhone 8 can still do it, and I’m really going to miss it when I upgrade.