Can you add text to the slide number in PPT 2019?

Yes, it is possible to add text to the slide numbers in PPT.

Go to View - Master - select the master slide (top left thumbnail). 

Click in the placeholder for the slide number and type your text.

Now go to Insert - Header and footer - check the slide number box and click Apply to all.

When you return to the presentation you will probably see that the slide number has not been updated. The text is not present.

You can try to reapply the layout using the Reset button in the slides group of the Home tab. If that does not work, go back to the master and select the layouts individually (notice that the text has not been added to the layouts) and uncheck the Footers option in the Master layout group and immediately recheck it. The text should now appear on the layouts. The text will still not be on the slides though, you need a last step.

Go to Insert - Slide number - uncheck Slide number and click apply to all. Then do the same and check the slide number option again and click apply to all. That should put things right. Not straigthforward I know but that is how it is.