Summary
Minecraft Hardcore's Anti-commands feature can be bypassed via LAN Connection. This is prominent in all existing versions of Minecraft that feature the hardcore difficulty.
Recreation Process
Enter an existing Hardcore world (or create a new one) > Pause Menu > Open to LAN > Allow Cheats: On > Start LAN World > You are now able to use cheats in Minecraft Hardcore
Thank you for your report!
We're tracking this issue in MC-134, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki