As for the existing implementations of CKEditor 5 with real-time collaboration, I'm afraid since the collaborative editor is usually a component of a larger platform (publishing, e-learning, CMS, intranet, documentation management system etc.) it'd be difficult to find something like a publicly available real-life use case demo of it. Without much effort, you can check out the real-time collaborative editing demos (https://ckeditor.com/collaboration/real-time-collaborative-e...) that we have on our website.
A while ago we started building our own internal implementation of a collaborative document management system using CKEditor 5 and features such as real-time collaboration with track changes (something like Google Docs) for our own needs - hopefully, once we polish it a bit, we will be able to release it. We are working on publishing a few case studies with existing customers, too.
As for offline collaboration, our current solution switches the editor to read-only when you e.g. lose connectivity, and then the editing mode is back on when you reconnect, along with the changes that were done in the meantime by other users when you were offline. The platform is ready to fully support "offline" writing. We tested some more complex scenarios with one of the users being offline - and it works (however, no periods as long as days of being offline). We focused on delivering other collaborative editing features such as track changes or comments that were more required by our customers. Full support for writing while being offline is still on our roadmap, though.
As for the answer to why the collaboration component is not Open Source: we have currently 40 people in total working on CKEditor (CKEditor 4 & CKEditor 5). Developing such a complex application takes a lot of time and resources and we simply need money to cover the expenses. Without that, we’d not be able to spend so much time on fixing bugs or bringing new features. Also, unfortunately, we learned the hard way that some of the biggest IT companies don’t want to help Open Source projects by spending even a single dollar on them, even if they use it in an application that brings them millions of dollars.
Regarding mobile support: CKEditor 5 works well on mobiles (with and without real-time collaboration). The comment you referred to was about the lack of an online demo of real-time collaboration on ckeditor.com for mobile devices. The reason behind this decision was that we are using the sidebar for displaying comments, which results in rather poor UX on mobiles. Fortunately, we are almost finishing the inline annotations plugin which will display comments inline, without using the sidebar. Feel free to assign a full Kiwi for mobile support :)
Thanks again for the article!