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The man page for GNU find states:

-exec command ;

    [...] The  string `{}'  is  replaced  by the current 

    file name being processed everywhere it occurs in the 

    arguments to the command, not just in arguments where 

    it is alone, as in some  versions  of  find.

    Both  of  these  constructions might need to be escaped 

    (with a `') or quoted to protect them from expansion 

    by the shell. 

That's from the man to find (GNU findutils) 4.4.2.

Now I tested this with bash and dash, and both don't need to have the {} being masked. Here is a simple test:

find /etc -name "hosts" -exec md5sum {} ; 

Is there a shell, for which I really need to mask the braces? Note, that it doesn't depend upon whether the file found contains a blank (invoked from bash):

find ~ -maxdepth 1 -type d -name "U*" -exec ls -d {} ; 

/home/stefan/Ubuntu One

This changes if the found file is passed to a subshell:

find ~ -maxdepth 3 -type d -name "U*" -exec bash -c 'ls -d {}' ; 

ls: cannot access /home/stefan/Ubuntu: No such file or directory

ls: cannot access One: No such file or directory

which can be solved by:

find ~ -maxdepth 3 -type d -name "U*" -exec bash -c 'ls -d "$0"' {} ;

in contrast to:

find ~ -maxdepth 3 -type d -name "U*" -exec bash -c 'ls -d "{}"' ; 

/home/stefan/Ubuntu One

but that's not what the man page is talking about, is it? So which shell treats {} in a different way?

Summary: If there ever was a shell that expanded {}, it's really old legacy stuff by now.

In the Bourne shell and in POSIX-compliant shells, braces ({ and }) are ordinary characters (unlike ( and ) which are word delimiters like ; and &, and [ and ] which are globbing characters). The following strings are all supposed to be printed literally:

$ echo { } {} {foo,bar} {1..3}

{ } {} {foo,bar} {1..3}

A word consisting of a single brace is a reserved word, which is only special if it is the first word of a command.

Ksh implements brace expansion as an incompatible extension to the Bourne shell. This can be turned off with set +B. Bash emulates ksh in this respect. Zsh implements brace expansion as well; there it can be turned off with set +I or setopt ignore_braces or emulate sh. None of these shells expand {} in any case, even when it's a substring of a word (e.g. foo{}bar), due to the common use in arguments to find and xargs.

Single Unix v2 notes that

In some historical systems, the curly braces are treated as control operators. To assist in future standardisation activities, portable applications should avoid using unquoted braces to represent the characters themselves. It is possible that a future version of the ISO/IEC 9945-2:1993 standard may require that { and } be treated individually as control operators, although the token {} will probably be a special-case exemption from this because of the often-used find {} construct.

This note was dropped in subsequent versions of the standard; the examples for find have unquoted uses of {}, as do the examples for xargs. There may have been historical Bourne shells where {} had to be quoted, but they would be really old legacy systems by now.

The csh implementations I have at hand (OpenBSD 4.7, BSD csh on Debian, tcsh) all expand {foo} to foo but leave {} alone.

The {} needed to be quoted in versions the fish shell prior to 3.0.0.

$ fish -c 'echo find -exec {} ;'

find -exec  ;

And in the rc shell (also akanga based on rc, but not es):

$ rc -c "echo find -exec {} ';'"

line 1: syntax error near '{'

Those are probably not the shells the authors of that GNU find documentation had in mind when they wrote that text since fish was first released in 2005 (while that text or similar was already there in 1994) and rc was not originally a Unix shell.

There are some rumors of some versions of csh (the shell that introduced brace expansion) requiring it. But it's hard to give credit to those since the first release of csh in 2BSD didn't. Here as tested in a PDP11 emulator:

# echo find -exec {} ;

find -exec {} ;

And the man page from 2BSD csh clearly states:

As a special case `{', `}' and `{}' are passed undisturbed.

So I would find it very strange if a subsequent version of csh or tcsh later broke that.

It could have been to work around some bugs in some versions. Still with that 2BSD csh (that's the same in 2.79BSD, 2.8BSD, 2.11BSD):

# csh -x

# echo foo {} bar

echo foo {} bar

foo {} bar

# echo `echo foo {} bar`

echo `echo foo {} bar`

echo foo {} bar

foo  bar

Quoting doesn't help though:

# echo `echo foo '{}' bar`

echo `echo foo '{}' bar`

echo foo {} bar

foo  bar

You could quote the whole command substitution:

# echo "`echo foo {} bar`"

echo `echo foo {} bar`

echo foo {} bar

foo {} bar

But that's passing one argument to that outer echo.

In csh or tcsh, you'd need to quote the {} when not on its own like in:

find . -name '*.txt' -type f -exec cp {} '{}.back' ;

(though that kind of find usage is not portable as some finds only expand {} when on its own).

In a word, csh. bash and other modern shells recognize that the user probably isn't asking for a null brace expansion. (Modern csh is actually tcsh and may also handle {} sanely by now.)