From what I can tell there is not a way to reference a page in the Modified() or ModifiedBy() formula even though the formula documentation indicates that you can reference the page object:
I did a few searches of the community and I see that other users are requesting the ability to reference pages as objects so that leads me to believe that perhaps the formula documentation is not correct in listing pages as a valid object. Perhaps @Al_Chen_Coda might have some better insight?
The thread I am referencing is here:
That being said - I am sure there is a solution to achieve your desired results. Is it possible for your use-case to use a table for input instead of directly on the page? If so, you can track when the row of a table is modified and can reference that on the page itself.
I have a doc for a daily updates from my team (think SCRUM standup). They enter their responses on a table - I have formulas set up on a page to make things look nice and include the date/time they submitted their answers to me (along with a button to email the page to myself). I know I can reference the page object using the button that emails the doc to myself but can’t seem to get a page object to show up in the Modified() or ModifiedBy() formula.