Before anything, you should find where the Autosave is and copy that file. The later in the game the better to avoid too much replay. In the very last level, there's actually a crucifix against a tree (as pictured below).

Anyway, follow the address below. AppData is usually hidden, but you can change this in Windows to show hidden folders.

User -> AppData -> LocalLow -> WZOGI -> Little Red Lie -> LRLAuto_0

The LRLAuto_0 is the Autosave as you can imagine, so copy this and pop in an extra folder that you can remember. This is just in-case anything happens during to the post-credits room.

If something does, simply take the copied file and paste it back in the Little Red Lie folder.

After finishing the final scene with Arthur, the credits will roll. Wait for them to finish and you'll see a glitched version of the first conference scene with Arthur. The Narrator will be present and will ask you what you're still doing here. This is similar to the previous confrontation with her so remember she has more control than you.

Answer with BOTH following responses. DO NOT select any other answer.

"There's still an achievement missing."

"Yes."

Continue talking and she will award you The Best Policy Achievement.

NOTE: If you keep talking after this, she will also manually unlock every other achievement you may have missed during your playthrough. So if you've reached 100%, congratulations!

If you continue talking to her, she will reveal she's destroyed your save file and blocked any return to this room. Meaning even if you replay the game from a new start, after the credits this room will not appear.

If you need or want the room to reappear, there is a workaround.

Anything dealing with the registry editor is very, very dangerous. Do not do anything other than what is mentioned in this guide.

Enter your Registry Editor through your Windows menu.
Follow the address below to the WZOGI folder and inside you will find another folder for Little Red Lie.

Computer\HKEY_CURRENT_USER\SOFTWARE\WZOGI\Little Red Lie

You can simply delete the Little Red Lie folder, or the whole WZOGI folder itself if Little Red Lie is the only game you have from them.

Once you've done so, relaunch the game. You should reload your most recent Autosave, so continue through to the credits. The post-credits room should reappear and you can answer the questions correctly as mentioned in the previous section.

As mentioned in the first section, make sure to make a back-up of your save if anything happens. If you don't have a save now because she destroyed it, you will have to fully replay the game after deleting the folder in order to return to the room.

Good luck.