This is more an "academical" question:
Is there a token which neither can be redefined to be \outer nor can be affected by \uppercase/\lowercase no matter what \lccodes/\uccodes are current?
Assuming that functionality of the \outer-primitive is available/does not get disabled.
A frozen \relax token more or less meets the description. As it is not a character token it is not affected by lowercase and it can not be redefined at all, any attempt would generate an error, or redefine the standard \relax
\edef\zz{\ifnum0=0\else\fi}\show\zz
%define \zzz delimited by a frozen relax
\expandafter\def\expandafter\zzz\expandafter#\expandafter1\zz
{\def\arg{#1}\show\arg}
% calling \zzz shows delimted argument is abc
\edef\tmp{\noexpand\zzz abc\zz}
\tmp
%using a normal \relax does not delimit the argument
\zzz abc \relax
\bye
\let\outer\relaxshould solve the problem ;-)\outerin my code. ;-) One token not to be refedinable in terms of\outerand not affectable by\uppercase/\lowercasewould be nice, e.g., with macros that insert sentinel-tokens for list-processing. The sentinel-token should not be transformable to s.th. else. The sentinel should not be redefinable in terms of\outeras that would break its usage with (internal) macros where it is inserted automatically as a compoonent of arguments. It is rather an academical/moot question.\(upp|low)ercasechange all character tokens (regardless of catcode, and assuming they have a proper\(u|l)ccode), so you are looking for a non-character token, which are control sequence (or symbol) tokens, which are all redefinable. (Discarding internal frozen TeX tokens, like\endwrite, that cannot be redefined, but can't be used in your code either).\outertokens can't appear in the argument to anything (including\(upp|low)ercase), so the answer to your question is “any already\outertoken”. This raises an error\outer\def\x{y} \lowercase{\outer\def\x{z}} \show\x, but still redefines\x...\letto\relax, so they aren't technically\outeranymore.