It seems virtually unsupported and extremely difficult to setup even for seasoned developers. Maybe that's by design to get folks onto the SaaS offering. Too bad it looked like a decent platform.
Like the title of this post points out, permit me to add that it woudl be sad to see this self-hosted project fade away especially at a time where more and more individuals are taking a genuine interest in "open Source" self-hosted stuff and slowly appreciating the work being done by open source projects. Having just quickly seen a few videos on this and comparing it to Bookstack it would be nice to see Booktype hold its own as it has so much to offer.
Thus, like the title asked would you mind me asking is this project dead? If not are there any plans to support more recent versions of Ubuntu?
Currently, active development of Booktype is stopped, but it does not mean that it's dead. It's open-source, it's developed using open-source libraries, so you can always research it, ask questions on a forum and send a PR if you want to improve it.
From my side, I can guarantee that your PRs will get full attention.
If you can move from self-hosted to the hosted version may be, the second concern of Deliverability can be addressed with new version of Mumara as well. Mumara has launched a new SaaS version of their email application naming it CampaignsPlus official website