It’s still there, but you have to tweak a dconf/gsetting or two. Easiest way to do that is with the dconf-editor which is a gui application to configure those settings. Not installed by default so go ahead and install it:
sudo apt install dconf-editor
Then in dconf-editor navigate to com->ubuntu->update-notifier->show-updates-status-icon. Flip the toggle for “Use default value” to off and then change the Default value to “True”. The editor will notify you a setting has changed and prompt you to reload it. Go ahead and do that.
The default for notification of available normal (non-security) updates is 7 days. You can navigate to com->ubuntu->update-notifier and change “regular-auto-launch-interval” to however many days you want to pass before being notified of normal updates. “0” sets it to as soon as they are available.
Security updates are handled by unattended-upgrades.