In short, the spell check feature in Excel 365 is like the old-style Word 2010 and older dialog.  Given that, I would assume it's like that in every version of Excel.  The spell check in PowerPoint is like the Word 2013 and newer task pane style spell check.

For the spell check feature in Excel:
1. Press F7.  It opens what will be familiar to users of Office 2010 and earlier as the spell check dialog.  For some reason the focus starts on the dictionary language.
2. Press alt+d to jump to "Not in dictionary".  This will show the first item found.
3. For reasons, it will read blank.  What is actually happening is the text is selected and it won't read properly.  Press an arrow key. We are aware of this and have an open issue: https://github.com/nvaccess/nvda/issues/11504
4. Press TAB once or alt+n to jump to the suggestions.
5. Use the arrow keys to move around the suggestions.

You can press either:
a) alt+i to ignore once
b) alt+g to ignore all instances of this error
c) alt+d to add the word to the dictionary
d) alt+c to change this instance to the currently selected suggestion
e) alt+l to change all instances of this error to the currently selected suggestion

So, aside from the initial quirk of where focus lands (would need a fix from Microsoft), and the oddity of NVDA not reading the misspelt word while it is selected, which we're aware of, the dialog isn't too hard to navigate.


For PowerPoint:
1) Press F7, the task pane opens.  Again for reasons known only to Microsoft, the focus is on the language selection.
2) Press tab once.  This moves the focus to the "Ignore once" button, which is the first button on the task pane.
3) Press the "Move to previous object" keystroke.  NVDA+numpad 4 (desktop) or NVDA+shift+left arrow (laptop).  This reads the error.
4) You can press enter to activate this button, or press TAB three times to move past Ignore all and add and to the first suggestion.
5) Use the arrows to move through the suggestions.

The options from here are identical to listed for Excel, except adding the word to the dictionary is alt+a rather than alt+d.  You can press either:
a) alt+i to ignore once
b) alt+g to ignore all instances of this error
c) alt+a to add the word to the dictionary
d) alt+c to change this instance to the currently selected suggestion
e) alt+l to change all instances of this error to the currently selected suggestion