It used to be that holding Option in Finder and then clicking the Go menu would reveal an entry for the normally-hidden Library folder. In macOS 10.12.3, for reasons unknown, Apple has changed this: The only thing the Option key does now is change Go > Enclosing Folder into Go > Enclosing Folder in New Window. To see the Library folder entry, hold down the Shift key instead.
So that’s the bad news: They changed an undocumented shortcut that many users have been told to use when troubleshooting. The good news is they added a better, easier, and faster way to get to the Library folder: Just press ⇧⌘L in Finder. This matches all the other shortcuts—⇧⌘O for Documents, ⇧⌘D for Desktop, etc. (Downloads is the odd one out, as it uses ⌘⌥L.)
Note: This may be a short-lived change, perhaps even a bug—it seems to be gone in the 10.12.4 beta release. Either that, or the 10.12.4 build is a bug, and it’s supposed to be how it is now in 10.12.3. Hey Apple, I have some advice on how to fix this whole messy situation: Stop hiding the Library folder by default! (Note that you can unhide it by showing View Options (⌘J) on your home folder and checking Show Library Folder.)