Self-updating textfield in Pages 6.2
This solution requires Numbers but the date changes on opening. 


In Numbers create a single cell table
Then enter the following formula: "Last edited: "&amp; DAY(TODAY()) &amp; " : " &amp; MONTH(TODAY())
select the table and copy
in Pages, paste the table in the desired location setting wrap etc.
format as desired


The day and month will update each time the document is opened.  To see the last time it was opened will not be possible unless you use Quicklook or get info on macOS.

Be sure to format the data as you wish it to be displayed. 

Formulas pasted from Numbers are preserved when pasted into Pages or Keynote. This work-around may be what you need until Apple re-introduces this missing feature. 

The example below was made in iOS, but the same applies to macOS.