Hey.

--passphrase-fd n
       Read  the passphrase from file descriptor n. Only the first line
       will be read from file descriptor n. If you use  0  for  n,  the
       passphrase  will  be  read  from STDIN. This can only be used if
       only one passphrase is supplied.

       Note that since Version 2.0 this passphrase is only used if  the
       option  --batch  has  also  been  given.  Since  Version 2.1 the
       --pinentry-mode also needs to be set to loopback.

However it seems that despite of version 2.2.27, --pinentry-mode loopback is not needed and it still works for me with e.g.:
gpg --quiet --no-verbose --no-greeting --batch --no-options --no-random-seed-file --no-default-keyring --keyring /dev/null --no-keyring --trustdb-name /dev/null --disable-dirmngr --passphrase-fd 0 --pinentry-mode loopback --no-symkey-cache --decrypt

--no-default-keyring
       Do not add the default keyrings to the list  of  keyrings.  Note
       that  GnuPG will not operate without any keyrings, so if you use
       this option and do not provide alternate keyrings via  --keyring
       or  --secret-keyring, then GnuPG will still use the default pub‐
       lic or secret keyrings.

First, AFAIU --secret-keyring is obsolete? Second, this is a bit ambiguous, it says without alternative --keyring it would still use the default keyring,.. but is this also the case when --no-keyring is used?

In past times I had to use --no-default-keyring --keyring /dev/null --secret-keyring /dev/null... my understanding was that now a simple --no-keyring would be enough?

Cheers,
Chris.