For those who migrated to Ubuntu 18.04 (Bionic Beaver) and got frustrated that the "Suspend" option disappeared, here is an interesting Gnome extension:
https://extensions.gnome.org/extension/826/suspend-button/
If you don't want to install it, the trick is to hold the "Alt" key so that the "Shut Down" option is changed to the "Suspend" one.
Not sure what is the rationale behind this UX design but for sure that did not work for me and the above extension made my day :)
Thanks https://extensions.gnome.org/accounts/profile/laser_b