Ref: reddit thread for same question. Thanks to u/kmt1980 for his help. Posting here on his behalf for others who run into this issue.
Plymouth requires Early-KMS init as mentioned in the Arch wiki. That is done by adding your VGA driver to `MODULES` in `mkinitcpio.conf`
edit `/etc/mkinitcpio.conf` and add the driver name to `MODULES=(...)`. My `MODULES` line looks like this
MODULES=(i915)
`i915` because Intel integrated graphics. You can find your VGA driver using this command:
lspci -v | grep -A10 VGA | grep driver
Finally run `mkinitcpio -p linux` to regenerate ramfs.