The last one I can find was Steve Carlton in 1980 who threw 304. For what it is worth the Innings leaders numbers before that typically ranged from 280 to 325. In the last 45 it has declined to more like 225 to 250. Roughly 60-75 less innings a year.
5 man rotations knocking 6 or 7 starts out of each frontline starter (33 starts vs 40) and bullpen usage are the obvious reasons. Pitch counts became an issue long after the trend was established.
I wish there was a way to know how many TJ would have been done in the 20's to 70's so there could be an apples to apples discussion of this topic.