Deprecated IEEE 1275 client interface functions

I've been looking at IEEE 1275 document that describes various client
interface functions for SPARC firmware.

The problem with SPARC bootblk is that it is limited by firmware to
around 8 KiB. Once the bootblk is executing, it needs to allocate a
memory segment, into which it can read a much bigger second stage boot
loader or kernel and then execute it.

I can use client interface functions like claim(), open(), seek(),
read() etc., in order to allocate a buffer and read my custom kernel
into it, but I hit an issue when trying to execute that code from the
buffer.

There is one control transfer function that is meant to execute another
program:

chain
  IN: [address] virt, size, [address] entry, [address] args, len
  OUT: none

Frees size bytes of memory starting at virtual address virt, then
executes another client program beginning at address entry. The
argument buffer args, len is copied into the Open Firmware memory and
passed to the other program. The address of the arguments in the Open
Firmware memory is the client program’s second argument, and their
length is its third argument. chain is used to free any remaining
memory for a secondary boot program and begin executing the booted
program.

This was failing for me and the reason was this function was disabled
in the firmware:

$ ack 'cif:'
...
obp/arch/sun4u/go.fth
153:cif: enter   ( -- )  reenter  call  ;
154:cif: exit    ( -- )  exittomon call  ;
155:\ cif: chain   ( len args entry size virt -- )  op-chain  ;

You can see on line 155 the '\' comment character disables this
function.

There is execute-buffer() in obp/os/bootprom/clientif.fth which can
execute ELF 64 code from a buffer allocated by claim(). I've tested it
and all seems to work, however I'm trying to understand the history
behind chain() and why it was deprecated.

I guess this may only be known to a person who actually made this
change to go.fth file years ago. However, if anyone here remembers the
reasons, please let me know.

Thanks.