The obvious problem there is an attacker who gets hold of the
initramfs in order to be able to use the credentials to request the
passphrase themselves. For those who wanted to get more elaborate
(and more likely to mess up and leave their server a useless
encrypted brick), there are things like Mandos:
https://wiki.recompile.se/wiki/Mandos
https://www.recompile.se/mandos/man/intro.8mandos
"This is the the Mandos system, which allows computers to have
encrypted root file systems and at the same time be capable of
remote and/or unattended reboots.
The computers run a small client program in the initial RAM disk
environment which will communicate with a server over a network.
All network communication is encrypted using TLS. The clients
are identified by the server using an OpenPGP key; each client
has one unique to it. The server sends the clients an encrypted
password. The encrypted password is decrypted by the clients
using the same OpenPGP key, and the password is then used to
unlock the root file system, whereupon the computers can
continue booting normally."