Apple is soon to release the latest version of macOS, 10.15 aka Catalina, and with that it replaces iTunes with 3 separate apps: Music, Podcasts and TV.
If you used to have half star ratings enabled in iTunes you need to re-enable those for the new Music app. To do so, open terminal and type (or paste) the following command:
defaults write com.apple.Music allow-half-stars -bool TRUE
Now, Music should (more or less instantly) show your half star ratings, again.