Flatten option is not available in Blender 2.92 I am following along a this tutorial at the mark 49:05 time, he is using F3 to open Search menu, and types FLATTEN.
When I do the same, the search results disappear, and I have no FLATTEN option.

I tried ShiftT as mentioned here but that did nothing.
How can I access the Flatten option?
I am not sure about any flatten option but if you have mesh:looptools enabled in preferences you can select your faces and then right click-&gt;looptools-&gt;flatten to get the result it seems you are looking for.