EndlessOS (started in 2017) is a distro based on debian that uses OSTree for an immutable root partition. It’s targeted at non-technical users and particularly suitable for not-always-online environments.
Flatpak is the preferred app packaging format, and Endless maintains its own collection of curated apps in the App Center.
I think it’s straightforward to install anything from Flathub as well, but it’s been a while since I last checked.