Is Calc missing the Excel manual outline feature?

Long ago, working with MS Excel, I frequently used a feature whereby you could outline a range of cells that contained no numeric values, and opt to outline the range MANUALLY, with buttons to “promote” or “demote” individual rows, and other buttons to hide or display various levels of your outlined information. This was especially helpful in creating Tables of Contents which could be modified instantaneously to hide lower-tier data, or reveal every row/column.

I haven’t had a need to use this feature since I retired 12 years ago. Now I’m working on a community project where such a flexible ToC would be valuable. Does anyone know if such a feature exists in LibreOffice Calc

Not sure if I understood your question correctly, but it seems to me that you are looking for the F12 key. It seems that over the past 12 years nothing has been missing from Calc that was useful then. It may have moved to another location, but it has not disappeared altogether.

F12 would be Data > Group and Outline > Group.

Group and Outline Help