Been going on for a long time. Many of the great earlier shows like the Rifleman, M*A*S*H, Have Gun, etc got ruined for me as they devolved more into 1960s liberalism. Global Warming wasn't an issue yet, though fossil fuel pollution was just getting heard about . . .
In addition to the shows you mentioned, creeping liberalism also helped kill
Bonanza. Even as a youngster, I remember asking my father "Dad - what happened to their guns?" (As I recall, Little Joe in particular - played by Michael Landon - was pretty handy with a sixgun.) And it got to the point where instead of running "squatters" off their land, they'd invite 'em home for dinner.
And it got to be pretty easy to tell when Alan Alda directed a M*A*S*H episode - because it sucked. He was - and still is - a dyed in the wool leftist.
A good part of the problem is that nearly all leftist-messaging shows are so fixed on the messaging that they're poorly written. Just off the top of my head, the only well written left leaning show that comes to mind was
The West Wing.