I mostly avoid autocorrupt because it would foul up my text, replace names with words or with other spellings of those names, and collect words I’d mis-typed even as I tried to re-type them…
I tried to use it to type 1/2 as a fraction today, but somehow, this deleted the 1/2 entry from the replacement table. I would need to type 1/2 as a fraction to restore that line of the table, and I can’t type 1/2 as a fraction.
I was able to repair that affected line by using the NeoOffice replacement table, and copying and pasting that character into the LibreOffice one. It’s crude, and I wouldn’t consider it a general solution.
If your desktop environment doesn’t allow you to type a 1/2 fraction or enter a U+00BD value to obtain the VULGAR FRACTION ONE HALF character (e.g. in GNOME it’s Shift+Ctrl+U then BDEnter), then you could in Writer use Insert → Special Character… and search for half or use numeric input, insert that character and copy to clipboard and then paste it in the AutoCorrect dialog, or copy-paste ½ from here or somewhere else.
Edit: if you set up Compose key handling in your desktop environment (you didn’t say which, in GNOME it’s in Settings → Keyboard, Special Character Entry, Compose Key), it’s simply ComposeKey12.