A family of Microsoft word processing software products for creating web, email, and print documents.
AFAIK this feature is available only in Excel for Windows. If you claim to have had it on a Mac it may have been available temporarily in Mac Excel on a test basis... Microsoft apparently has a back door through which they can slip stuff in & out to small pilot groups to monitor the feasibility of a new feature.
From Microsoft KB:
Use Insert Controls to add rows and columns
Insert Controls work only with a mouse. If you’re using a touch device, use the mini toolbar, described above.
Insert Controls appear outside your table when you move your cursor just above or to the left of two columns or rows. Click the Insert Control, and a new column or row will be inserted at that location.
