Godot Engine is an open-source game engine which is distributed under the MIT License. This licensing ensures that the engine can be used freely for any purpose, commercial or non-commercial, and the source code can be modified or redistributed without concern for royalties or licensing fees.
The development of Godot is supported by the Software Freedom Conservancy, and it relies on community contributions as well as donations from users and sponsors. The MIT License and the governance of Godot by a not-for-profit entity both strongly suggest that Godot will remain free to use indefinitely.
However, while the current version and all of its derivatives under the MIT License will remain free, there is no absolute guarantee regarding the future. Circumstances could change if, for example, the project governance were to change or a fork were to be created under a different license.
In summary, as long as Godot remains under the current governance and maintains the MIT License, it will continue to be free for anyone to use, modify, and distribute.