By the 1990s, you didn't need AC, but your home/rental was more appealing if it had one, because there were a few hot days a year that were pretty uncomfortable otherwise.
Now, you can't not have it. There are far too many hot days to live without it.
- cherry blossoms have been consistently blooming earlier each year
- some areas have been breaking historic high temperatures over the past 3 years (e.g. 伊勢崎市)
- even this year, there were several 20C days in Tokyo where the climate felt more like spring than winter
- 気象庁 is surveying for a new word to describe days with temperature exceeding 40C, since they are now becoming common in some areas.
Lastly, one joke my friends say is "In Japan there are four seasons: rainy season, summer, midsummer, and winter."
Now every summer day is 30c+.
Also, a comment I hear often is that people didn't really need air conditioners back then. You definitely cannot get away with living in Tokyo without an air conditioners these days!