What is the Numbers toggle to freeze and unfreeze panes when viewing data?

If you are talking like Excel does it, there is no freezing/unfreezing of panes. In Numbers you may have up to five header rows at the top and five header columns at the left. You can freeze or unfreeze these header rows and columns. To do so, right click on the column letter or row number. It is in the contextual menu that pops up. You cannot freeze any other rows or columns

Here's another way, via the menu:

As with any other menu item in apps running under macOS, you can assign a keyboard shortcut to any menu item that doesn't already have a shortcut Then that shortcut becomes similar to a toggle.

SG

Numbers allows you to freeze and unfreeze panes by designating them as headers or footers. To do this, select the table in which there are panes you'd like to freeze. Then, in the upper-right corner of the Numbers window, select 'Format'. This will open a sidebar with four headings, 'Table,' 'Cell,' 'Text,' and 'Arrange.' Select 'Table' and look for the section titled, "Headers & Footer." There are three drop-down controls corresponding to header columns, header rows, and footer rows (in that order). Select the appropriate dropdown and select the number of column headers, row headers, or row footers you'd like to effectively freeze. When you do so, the headers will be highlighted according to the selected table style. In the example image I've provided, the headers (frozen column and row) are shaded gray. I hope this answers your question. Good luck!