this is a design concept thing, I guess. On one side I understand to have a clear design in menu sequences, on the other side it was somehow logical to find the button for generating TOC in the window. This is more understandable for those who don't work often with Sigil. Due to the fact that ebooks will be converted and more often than have problems with the TOC, a lot of people are looking for this and found it directly.
What was an interesting experience for me: I had recognized the missing button immediately - before I had seen something else of Sigil.
But anyway, I can work with both solutions