Decades ago, the rule of thumb was that there were no rattlesnakes above 9,000 feet in the Sierra. I don't remember what the authority for this was. It is definitely warmer in the Sierra now than it was 20 years ago. Is there such a rule of thumb now, and if so, what is the rattlesnake line? This question was prompted by Venomous/Semi-Venoumous Snakes at Higher Altitudes
This isn't a scientific evidence based answer, but I think that line should not be changing much, as it should have more to do with snow cover than temperature.
This article on the history of snow fall in the Sierras, asserts that there has been no change in snowfall over 130 years despite a change in temperatures. This isn't too surprising as air carries more moisture when warm, and the Sierras are known to getting a lot of powder (aka Sierra cement) days late in the season.
Also note from the chart that there aren't very many 'average' years, it's feast or famine. The years with heavy snowfall might mean six months of snow cover, maybe that's too long for a rattle snake to wait for breakfast?
Related trivia: The Tahoe basin remains rattle snake free (last time I checked) despite being theoretically in their range. The lake is about 6200 feet, with many passes well below 9000 on all sides, plus the Truckee River canyon going right down to known rattle snake territory (Reno). In a heavy snow year virtually all of the basin will have snow cover, nearly down to lake level, from December to March, with more snow in April and May.