Automatically create monthly Photo Stream albums in the new photos app
What was created in iPhoto was an Event for each month's worth of Photo Stream photos. Since the concept of Events no longer exists in Photos, that automatic creation has gone away too.

You could manually replace them with Smart Albums. There are a couple of different ways you could approach it:

Create a new Smart Album for each month. This can be done ahead of time, with the future months' albums just sitting empty until photos with those dates appear. Settings for this would be:


"Date"
"is in the range"
"5/01/2015"
"5/31/2015"


Obviously changing the dates for each month.

You could also create one Smart Album that just shows the last month's worth of photos. It wouldn't always align to calendar months, but instead just show the last 28 days worth of photos. Settings for this:


"Date"
"is in the last"
"1"
"months"