They were ambigous about future plans and still are. But in short term it seems to remain closed. I personally find it troublesome for several reasons. Harmattan mistakes shouldn't be repeated (unusable code since it's closed when the platform is abandoned), no effort sharing and etc. Nemo for example can't benefit from core applications of Sailfish if they'll remain closed.

In the best case, it would be good to get Jolla to express a will to open stuff eventually and to make a roadmap for that. In the worst case it will remain closed.