It appears in /isolinux/menu.cfg in debian live-usb iso.
APPEND initrd=/live/initrd.img-4.19.0-5-amd64 boot=live components splash quiet
This doesn't seem to be a kernel parameter. I wasn't luckier in debian documentation either. The genericness of the term has defeated my google-fu.
This menu.cfg file being generated hinders search as well.
The "components" boot parameter is provided by the live-config package on Debian. The live-config(7) on Debian Manpages had mentioned this boot parameter.
Boot Parameters (components)
live-config is only activated if 'boot=live' is used as a boot parameter. Additionally, live-config needs to be told which components to run through the 'live-config.components' parameter or which components to not run through the 'live-config.nocomponents' parameter. If both 'live-config.components' and 'live-config.nocomponents' are used, or, if either one is specified multiple times, always the later one takes precedence over the previous one(s).
live-config.components | components
All components are run. This is what live images use by default [...]
Debian Live CD uses such boot parameter by default and usually specified in boot/grub/grub.cfg and isolinux/menu.cfg of the ISO image (naming and path may vary).
The boot parameter boot=live components is also used for other Live CD variants, which can be found as DebianLive/MultibootISO and BootUsbWithGrubRescue on Debian Wiki.