Until Google forced the new calendar on users, I was able to hide selected morning/night hours using a Labs feature. Is there any way to do the same in the new version, either through the calendar itself (preferable), a browser extension, CSS sheet, or another method?
Since no one wanted to provide an actual solution, and somehow the standard Google response got accepted as the answer, this extension will allow you to change the working hours of Google Calendar, and more. This will work for the Material redesign of GCal.
Google allows G Suite users to set "Working Hours" From General settings. However, normal GMail users do not have this same functionality. I've tried the extensions above and they do not see to be effective - all they do is force the "Collapsable Day" function that hides the hours when your browser window size is reduced. It isn't worth the unnecessary extension that can hide malicious behavior.
The best answer is to adapt to the current software structure and provide feedback to Google via the feedback form. This is what leads to UI development and updates.
There is a browser extension, Google Calendar Plus, maintained for Chrome that does this well.
Opening the GCalPlus options via the puzzle icon you see, e.g.:
At the bottom you will see the X am to Y pm option which limits visibility on the hours.