I don't think there is anything "left wing" about a "Racism is bad" themed episode, unless you buy the leftist propaganda that conservatives have always been racist and/or anti-civil rights, and all those card-carrying RAT politians opposing desegregation in the 1960s were like "today's Republicans" and that "the two parties switched sides" overnight by magic.
In any case, the episode Let That Be Your Last Battlefield was indeed an example of social commentary in TOS, though it wasn't "very good science fiction", nor an example of TOS's finest hour.
There was a thread a while back on TV episodes/movies that were filled with SJW propaganda but were nevertheless entertaining as hell. Such a phenomenon is rare but it does occasionally happen, like Joel Schumacher making a great movie.
One of Jodie Whittaker's early Doctor Who episodes is like that, she goes back in time to fix history so Rosa Parks will stand up on the bus. The episode was a blast to watch, even though it was obvious SJW propaganda. NOT because it dealt with "racism", but because it was obviously made to thumb its nose at the Trump era.
That makes it a "blast"?
Anyway the villain's actions were pointless, if it wasn't Rosa Parks it would have been someone else. In fact before Parks there was 15 year old Claudette Colvin, who didn't get much publicity.
I don’t think I’ve seen “Let That Be Your Last Battlefield “
I had heard of it, that it sucked. But the Hollywood reporter ranks it #11 of TOS episodes so I guess some people like it.
Frank Gorshin was the highlight of that heavy-handed ST ep, however.