There's a need for a separate boot target since
persistent storage is way slower than tmpfs indeed;
usbflash has a tendency for huge performance drops
given simultaneous writes in addition to reads which
are the bottleneck already.

make-initrd-propagator 0.18 introduced ext4 rw slice,
so the corresponding kernel module needs to be included
into stage1; see also #28289.

NB: not available on x86_64-efi (or hybrid GPT to be strict)
due to fragility of the hack being made: parted(8) panics
upon seeing that, and good ol' fdisk is unable to treat it.

NB: use/live/rw use/rescue/rx use/syslinux/ui/gfxboot
are unlikely to play very nice together due to the latter's
magic l10n: "session" label is taken by live_rw config snippet
and *is* translated in design-bootloader-source;
OTOH "rescue_session" is *not*.