How to disable WordPress from automatically changing “x” to multiplication symbol “×” when typing numbers that also contain an “x” between them?

This is part of wptexturize, which includes a number of other transforms such as generating en- and em-dash symbols, ellipses and curly quotes. (Here’s the specific code for this, known as ‘multiplication symbol’ in the documentation but ‘times’ in the code.)

You can disable wptexturize completely with

add_filter( 'run_wptexturize', '__return_false' );

but there’s no built-in way to disable just this one wptexturize rule.