Yes, you can still install fonts by moving or copying them into the user's Library/Fonts folder, without recourse to Font Book.
Rebooting should not be necessary, though most apps will need to relaunch to update their font menus.
They can even be inside subfolders, if you want to organize them.
macOS supports most modern font formats: ttf, ttc, otf, dfont. Apple described Type 1 PostScript fonts as ‘legacy', which should be a warning that support will be removed at some point.
Symlinks to fonts in other locations may not work.