Math-Mode: No-Delimiter

When \left. is used, TeX inserts no delimiter, but adds a space of width \nulldelimiterspace. It's not advisable to set this parameter to zero, in my opinion. But with

\newcommand{\noleft}{\left.\kern-\nulldelimiterspace}

using

\noleft x\right|_{a}

will in any case remove the added space.

As explained in my answer to the referenced question, using a macro would be much better for ensuring consistency. If your aim is to get code that works also with MathJax out of the box, you're on the wrong road: the capabilities of MathJax are limited and so you are forcing yourself into using bad LaTeX.

<stuff>\big|_<index>

is also possible. There is also \Big, \bigg, and \Bigg