Note that Outline's definition of "open source" diverges quite a bit from the one that most people have in mind when they hear the words "open source".

From the text of Outline's license [1]:

    Notice

    The Business Source License (this document, or the “License”) is not an Open
    Source license. However, the Licensed Work will eventually be made available
    under an Open Source License, as stated in this License.

I think it is more accurate to say that Outline will be open source, rather than Outline is Open Source.

[1]: https://raw.githubusercontent.com/outline/outline/develop/LI...