“init_boot.img” is missing from latest 3.7 and 3.6 recovery-IMG-e-3.[67]*.zip - but they are available in recent LineageOS builds and are referenced in install documentation.
I already search the forum but didn’t find a clear/proper solution.
Personally I put it down to a packaging problem but there was never a clear explanation why this is omitted on some devices (sometimes perhaps missing routine for newly supported devices). My preferred solution is to look for and extract init_boot.img from the ROM itself with payload-dumper-go as described [Walkthrough] Extract eRecovery and boot images using payload-dumper-go.
Alternatively borrow a Lineage one, of same Android version.