If you're looking for an actively developed Maemo rebased on top of modern Debian, you might be interested in Maemo Leste. [1] It's actually rebased on top of Devuan Beowulf rather the Debian (Buster), but they are open to having a Debian flavour with systemd too if anyone is willing to add support for this.

[1] https://leste.maemo.org/

maemo felt more like a specific initiative, with a core stack of libraries & cross-inregeative apps being built atop them.

I get the impression Mobian is more like Debian itself. anyone is welcome, whatever tech you have.

came to look for this comment. Maemo -> Meego -> Mer -> Sailfish.... now this. I guess this one runs like a un-opinionated Debian but for mobile.