Tried testing the issue on reported chrome version #73.0.3642.0 using Ubuntu 17.10 and Ubuntu 14.04 by following steps as per comment#0. Observed that the shortcut "alt+p" is not working to pin or unpin the current tab.
Note: Tested the issue on chrome version #60.0.3112.113, observed the same behaviour there as well.

@reporter: Could you please confirm if this is a feature request, if not requesting you to confirm if the issue is specific to Ubuntu 18.04 so that it would be really helpful in further triaging of the issue.
Thanks.!

Chrome doesn't define an Alt-P shortcut nor any other pinning-related shortcuts:
https://cs.chromium.org/chromium/src/chrome/browser/ui/views/accelerator_table.cc
https://cs.chromium.org/chromium/src/chrome/app/chrome_dll.rc

Check chrome://extensions/shortcuts
Maybe Alt-P is listed there in which case it's an extension you have.
Otherwise it could be something provided by your desktop manager or a system utility.

Apologies, I'd installed an extension that adds the shortcut.

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

@Reporter: From comment#3, #4 and #5 it is understood that there isn't any issue with chrome. In that case could you please let us know if this can be closed.

Thanks!