Julia source code is encoded in UTF-8, so only Unicode characters are supported. There’s no markup language layer on top of the raw unicode characters. If we had this, most text editors wouldn’t be able to render Julia source code!
Most letters and numbers are available as unicode subscript characters, so in this particular case you can just write foo₁₂₃.
This needs to be typed as foo\_1<TAB>\_2<TAB>\_3<TAB> though, which is quite inconvenient.
Actually with this in mind it would be a cool extension to the Julia REPL to allow a tab completion of this identifier as the latex-like foo\_{123}<TAB>. (If this was in the main REPL, the various editor plugins would also probably support it fairly quickly.)