In my opinion, watching old tapes of her matches, and reading documentaries of her career, Austin started to show her first slight signs of decline in 1981, the first year she also started to deal with serious injuries. It may sound odd I am saying I believe she was actually starting a decline in 1981 when she won a slam title in 1981 and not 1980. However in 1980 I think she was regarded by most as the best player in the World, she just got unlucky to lose 3-setters to a truly inspired Goolagong in the 1980 Wimbledon semis, and a a truly inspired Evert in the 1980 U.S Open semis. Austin was really picked to win both matches, and both events in fact, and both those great players really had to "play up" if anything to take her down. Austin also may have been over-confident going into both those matches. Plus she chose not to play the French that year, at a time top players didnt all play that particular slam, while Evert did and won, which led to her ending that year #2 ranked.
However in 1981 while she did win a slam-the U.S Open, she got pretty lucky as her draw ended up with none of her main rivals to face to reach the final. Evert and Navratilova were in the other half, Jaeger was upset, Mandlikova did not end up in her path, nor did Shriver. She then got very fortunate when Navratilova choked badly in the final. One sign of her decline is losing in the quarter of both Wimbledon and the Australian Open in straight sets to Pam Shriver. She had been 9-0 vs Shriver before those back-to-back straight set losses. Now despite Austin beating Shriver 9 times in a row at that point, I would not just state with certainty losing to Shriver shows she was in decline or a slump, since I know Pam was a quality player of that era. However it is pretty strange to lose two times in a row in straight sets, in consecutive slam quarters as turns out, to a player your own age you had beaten 9 times in a row. Plus watching her on tape she just looks a bit weaker to me that year then she had before, not quite as strong, not quite as sharp in her play.
1982 is when she really shows a big decline in her level though. She doesnt even look like the same player anymore, even though she mantains a credible ranking. Then 1983 she shows another big dropoff. I can see watching her on tape why she retired in early 1984.