i dont think that unlocking bootloader voids warranty.
You can reflash to stock rom any time and relock your bootloader.
That's what seems to happen to all ROMs by @SebaUbuntu. Check https://files.sebaubuntu.me/ROMs/LineageOS-17.1/.Ok nice LineageOS... but... where are the updates for https://forum.xda-developers.com/re.../rom-lineageos-17-1-redmi-note-9-pro-t4144561 ?
After 1st release it was abandoned without notice...
fastboot erase userdata
fastboot flash recovery recovery.img fastboot reboot recovery
adb sideload <ROM filename>