What Chinese characters are rare in modern but common in classical Chinese?
There are people studying this.

Classical Chinese Character Frequency List

Modern Chinese Character Frequency List

Now do your own comparisons because I'm lazy.



Update -- I feel less lazy today so I'll give it a shot.

For obvious purposes, let't take the first 1000 characters in the classical list and see where they go in the trend. I can't do scripts so I'll do excel. Anyway, here are words that are falling out of favor:


Ancient date and time system: 甲乙丙丁, 子丑寅卯, etc
Ancient bureaucracy: 丞, 诏, 诛, 朕, 卿, 吏, 郡, etc.
Some daily words are also fading away: 吾, 汝, 焉, 哉, 勿, 曰, etc.
Moral concepts: 孝, 贤, 贞, etc.
Hard to categorize: 骸, 淮, 朔, etc. Either the concepts are no longer popular, or there are substitutes that is becoming popular....