What makes a Linux distro light?
Raspberry Pi Desktop is lightweight because it's an x86 version of a brutally pared-down Debian originally meant for a single-core Arm computer with 512MB of RAM.
Bodhi GNU/Linux is lightweight because it's Ubuntu but with all the desktop stuff removed, replaced with a forked old version of a very lightweight window manager and almost nothing else. Any functionality you want you must install.
Lots of different answers, lots of different use cases, lots of different strategies.
This is not a "yes/no" question. It's complex and nuanced.
Debian is not lightweight. Its strapline is "the universal operating system". It's a Swiss Army knife that can do anything and that's part of its definition.
You can make a lightweight install of it if you know what you're doing but ticking the box for a lightweight desktop and installing is not doing that.
Comparison: you see a lightweight sports motorcycle. It's green. You buy a Harley and paint it green and say "look mine is a lightweight sports bike now!"
Devuan is just Debian with systemd removed and openrc or sysvinit in its place. This is not a big sweeping change. It's equivalent to looking at the sports bike, seeing it has Bridgestone tyres instead of Dunlop, and swapping the tyres on the Harley to Bridgestone tyres.