[SanderSantema]

⌘⇧5 (CMD + Shift + 5) doesn't do anything on my macOS 10.13.6 system, ⌘⇧⌃4 (CMD + Shift + Ctrl + 4) is the default shortcut to take a rectangle screenshot and store it in the clipboard.

[runjake]

⌘⇧5 was introduced back in 10.14.0 Mojave.