Cast sequences don’t work in Classic like they did in vanilla and tbc because we’re using the legion macro system that was changed. If a spell or ability or item use is on the global cooldown, then it can only be placed at the end of a macro. Once the GCD activates the macro will stop functioning and nothing after it will happen. (This is also why the hunter 1 button rotation macro will not work in Classic TBC.)
But you can do modifier macros. Say you want shadow word pain, mind blast, and your shield in a single keybind you could macro this;
#showtooltip
/cast [nomod] shadow word: pain
/cast [mod:shift] mind blast
/cast [mod:alt] power word: shield
If you just press the keybind it will SW:P, if you hold shift when pressing it it will cast mind blast, and if you press alt while hitting the keybind it will cast your shield.