Supreet has found another issue related to this:
------------------------------------------------
On RHEL 6.7 : 

[root@localhost ~]# set myvar = `\echo "1 2 3"`
[root@localhost ~]# echo ${myvar}
1 2 3
[root@localhost ~]# echo ${myvar[1]}
1
[root@localhost ~]# echo ${myvar[2]}
2
[root@localhost ~]# echo ${myvar[3]}
3
[root@localhost ~]# echo ${myvar[3]}
3
[root@localhost ~]# echo $SHELL
/bin/tcsh
[root@localhost ~]#

On RHEL 6.8 :

[root@vm6 ~]# set myvar = `\echo "1 2 3"`
[root@vm6 ~]# echo ${myvar}
1 2 3
[root@vm6 ~]# echo ${myvar[1]}
1 2 3
[root@vm6 ~]# echo ${myvar[2]}
myvar: Subscript out of range.
[root@vm6 ~]# echo $SHELL
/bin/tcsh
[root@vm6 ~]#

This gives the correct behaviour
  set myvar = `echo "1 2 3"`

But this doesn't in 6.8
  set myvar = `\echo "1 2 3"`

I found these comments in the change log, it looks like it did more then preserving the backslash.
- quote backslashes properly to preserve them in `...` expressions
Resolves: #1301857

The real reason I came across this issue, is that there is a widely distributed setup for
shell completion support for git that fails with the 6.8 tcsh.
https://github.com/git/git/blob/master/contrib/completion/git-completion.tcsh

This is the code...
set __git_tcsh_completion_version = `\echo ${tcsh} | \sed 's/\./ /g'`
if ( ${__git_tcsh_completion_version[1]} < 6 || \
     ( ${__git_tcsh_completion_version[1]} == 6 && \
       ${__git_tcsh_completion_version[2]} < 16 ) ) then
        echo "git-completion.tcsh: Your version of tcsh is too old, you need version 6.16.00 or newer.  Git completion will not work."
        exit
endif
--------------------------

I have already informed upstream about this:
http://mx.gw.com/pipermail/tcsh-bugs/2016-May/001049.html

Upstream has already fixed the regression, here is the commit:
https://github.com/tcsh-org/tcsh/commit/819ab9146f10a3

*** Bug 1347237 has been marked as a duplicate of this bug. ***

I have closed this by mistake (thought this was fixed in RHEL 6.7.z), reopening...

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2017-0731.html