Once you get your 20 year retirement letter and want to stop being active, do you have to retire or can you go IRR and keep earning points?

Once you retire reserve or retire discharge you don't earn any more points.
If you retire Reserve, you remain available for recall and continue to have pay posted annually to your retired grade of rank.

I'm not sure why Rallypoint didn't show me this until now, rather than a year ago when you posted it, but since you haven't gotten an answer yet, here it is:

Once you have your 20 year retirement letter, you CAN go into the IRR and continue earning points. Note, however, that there are various provisions of law and regulation under which you can be forced to retire: reaching the statutory maximum time in service for your rank, for example. One important one that the Navy has, which you should look to see if there is an Army equivalent of, is that a member can be involuntarily discharged (or retired, if they already have their 20 year letter) if they don't earn a specified minimum number of points in a given year.