It is bundled neither zsh sources nor zsh-completions, so I wrote it.
#compdef screencapture __screencapture_files() { for ((i = 2;i < CURRENT;i++)) do if [[ $words[$i] == "-p" ]]; then return fi done _files } _arguments \ '-c[force screen capture to go to the clipboard]' \ '(-i -J -s -w -W)-b[capture touch bar only non interactive modes]' \ '(-b -i -J -s -w -W)-C[capture the cursor as well as the screen only in non interactive modes]' \ '-d[display errors to the user graphically]' \ '(-b -C)-i[capture screen interactively. ctrl clipboard, space toggle mode, escape cancel]' \ '-m[only capture the main monitor]' \ '-D[screen capture or record from the display specified]:display_number:' \ '(-b -s)-o[in window capture mode, do not capture the shadow of the window]' \ '-p[screen capture will use the default settings for capture]' \ '-M[screen capture output will go to a new Mail message]' \ '-P[screen capture output will open in Preview or QuickTime Player]' \ '-I[screen capture output will open in Messages]' \ '-B[screen capture output will open in app with bundle ID]:bundle_id:' \ '(-b -i -w -W)-s[only allow mouse selection mode]' \ '-S[in window capture mode, capture the screen not the window]' \ '-J+[sets the starting of interactive capture]:style:(selection window video)' \ '(-p)-t[image format to create default is png]:format:(pdf jpg tiff gif)' \ '-T+[take the picture after a delay of seconds]:seconds:' \ '(-b -i -s -W)-w[only allow window selection mode]::->interactive' \ '(-b -i -s -w)-W[start interaction in window selection mode]::->interactive' \ '-x[do not play sounds]' \ '-a[do not include windows attached to selected windows]' \ '-r[do not add dpi meta data to image]' \ '-l[capture this windows ID]:window_id:' \ '(-b -i -s -w -W)-R[capture screen rect]:rect:' \ '(-b)-v[capture video recording of the screen]::->video' \ '-V+[limits video capture to specified seconds]:seconds:' \ '-g[captures audio during a video recording using default input]' \ '-G[captures audio during a video recording using audio ID specified]' \ '-k[show clicks in video recording mode]' \ '-U[Show interactive toolbar in interactive mode]' \ '-u[present UI after screencapture is complete]' \ '*:files:__screencapture_files'