By the 1990s, it was still common for classic segments to be shown. I don't think very many segments were officially taken off the show (I wonder if Sesame Workshop has a box or vault for segments that are not allowed to be shown). But I think by the 1990s that there were so many segments that it would have been hard to show everything (that wasn't banned or dated, or otherwise had any broadcasting restrictions) on a regular basis. Segments were still being repeated, and I guess the producers wanted newer stuff to be shown quite a bit, but there were so many sketches that some were bound to be absent from an entire season, or only shown once. I guess skits about certain letters or numbers could have been repeated more. Others could have been guranteed depending on what learning topics were used, and if there were multiple plots with the same type of topic (like if there were mutliple stories about baths, then something like Rubber Duckie or Do De Rubber Duck would probably have a chance to be repeated throughout the season).
Now, I guess it would be a surprise if anything is repeated in the same season, since there are only 26 episodes a season. Up to five number of the day segments could be repeated in the same season, but there are multiple number of the day segments for different numbers. Each letter of the day segment can only be shown once a year, with there being 26 letters and 26 episodes (I've heard that that's the main reason why there are 26 episodes a season, instead of 25).