A couple of comments here. Ultimately, whether or not Space Mountain is open on any particular date is subject to change. However, when an attraction is closed, it would be explicitly listed in advance under the "Closed for Refurbishment" section on the applicable dates' entertainment schedule/park hours page(s). Here's an example when Indy is closed on June 12th:
https://disneyland.disney.go.com/calendars/day/2018-06-12/. Also, the ride itself would have a major disclaimer on the attraction page, such as Indy does here:
https://disneyland.disney.go.com/attractions/disneyland/indiana-jones-adventure/.
I find it hard to believe a major attraction such as Space Mountain would be closed during early summer, absent express prior notice. I think the "times unavailable" issue noted by several posters simply refers to Hyperspace Mountain. Disney probably hasn't decided whether they'll keep the overlay or go back to the original version. I don't believe a prolonged refurb (or any refurb) is on the docket, except the one day potentially needed to switch over.
Many attractions, such as Toon Town character meets and the Mark Twain are listed under "Schedule Unavailable" here on June 4th:
https://disneyland.disney.go.com/attractions/disneyland/ (you can toggle dates on the top right). Pirates is actually closed and listed as such.
I'm hoping for an extension of Hyperspace Mountain myself.