We can lock a map frame on the layout in Pro. But it's not about that.
I often make a map template that I then will re-use on multiple layouts in the same projects. They will have different layers added/removed, off/on, etc. For this I'll create and modify multiple maps from that single map template. Occasionally, however, I'd forget to make a copy of the template and modify the template itself, thus breaking the carefully crafted map. We need to have a way to lock the completed map itself, so no one can violate it, unless unlocked.