You can purchase the current version of SpaceEngine on Steam or GOG . How to Get SpaceEngine for Free (Legacy Version)

Where to download legacy versions like 0.980? : r/spaceengine

If you cannot purchase 0.990, you can still download the older, official freeware versions. All versions prior to 0.990 are intended to remain free.

In June 2019, developer Vladimir Romanyuk released version 0.990 on as a paid beta. This shift was necessary to fund more intensive development, such as:

A massive graphical overhaul for deep-space exploration.