Japanese text can be written with some fifty kana, which can be enciphered with substitution ciphers. (Historical examples can be seen e.g., at another article). The Chinese language employs thousands of characters, so any practical encryption in Chinese is based on numerical code of those Chinese characters.
What about Korean? I have long wondered whether substitution cipher is possible in Korean script (Hangul). This is because Hangul text is not simply a series of Hangul letters. Instead, two or three Hangul letters (consonant+vowel or consonant+vowel+consonant) are combined to form a syllabic block, which is the unit of writing (morphemic block). Moreover, placement of letters within a block differs depending on the vowel. The vowels for A, E, I (which have a vertical axis) are written to the right of the initial consonant, but the vowels for O and U (which have a horizontal axis) are written under the initial consonant. So, simply substituting one Hangul letter for another may result in a letter sequence that cannot be formed into morphemic blocks.
Googling finds apps to use cipher in Hangul, but I still cannot find historical examples of substitution ciphers in Hangul. Considering that Hangul was not used in official documents until 1894 and that I have seen use of numerical codes for telegraphy (posted here), substitution in Hangul may not have been common historically.
The following describes four possible schemes for substitution ciphers to work in Hangul, of which I found the third in a book for kids. The second and the third options require use of non-Hangul script (e.g., Arabic figures or other graphic symbols) for the substitution alphabet.
Substitution may be applied in separate groups for consonants and vowels. This way, the "consonant-vowel" grouping is preserved in substitution. This is the approach that AI (Copilot) on my PC gave to my question. Actually, AI at first gave an example of a Caesar cipher of consonants alone. When I asked about vowels, AI gave me a separate substitution table for vowels. I do not know whether this simple scheme really works for actual Hangul text. (For example, I hear not every consonant can be the third element in a block; there are letters that are not simple vowels or consonants.) To my repeated requests to give actual examples, AI simply ignored "in Hangul" and gave websites for the Caesar cipher in Japanese!
Substitution may also be applied to morphemic blocks rather than to letters. Unicode registers 11,172 Hangul morphemic blocks (399 consisting of two letters + 10,773 consisting of three letters). If we think this whole set as a single alphabet, substitution cipher can be used. (An attempt to use the Vigenere cipher in Hangul (pdf in Indonesian?) seems to use this approach.)
A simple practical solution is to provide for symbols for two-letter combinations (V+C) and also symbols for individual consonants. A C-V-C block can be enciphered with two symbols for CV and C. For example, 한(h-a-n) may be enciphered as 하(h-a) and ﾤ (n).
I found this scheme in a Korean translation of a book for Japanese kids, Yutaka Hara, Kaiketsu Zorori, vol.11, "Kaiketsu Zorori and the Mysterious Aliens" (Japanese: かいけつゾロリのなぞのうちゅうじん, Korean: 쾌걸 조로리, 수수께끼의 외계인 (Wikipedia in Korean)). The story does not involve a cipher, but is purported to be originally written in "alienese" (Japanese: うちゅうご, Korean: 우주어). The back endpaper prints a table for reading alienese, which is a substitution cipher table. The Japanese table, also used for riddles for kids, can be found at the publisher's website (here and here). The Korean version is an adaptation of this to the Hangul syllable system.
To look more closely, the substitution symbols for two-letter blocks in Korean are systematically designed as a combination of two graphic symbols for a consonant and a vowel. The graphic symbols for consonants can be used alone to represent for the third letter in a CVC block (e.g., ﾤ in 인).
The table also gives alternative symbols for three vowels, which I suppose may be used for double vowels. (It should be remembered that Hangul has a consonant symbol for "no initial consonant".)
The table includes aspired consonants (e.g., ㅋ) (of which the Hangul letter has an additional stroke) as well as plain consonants (e.g., ㄱ), while it does not include tense consonants (e.g., ㄲ). Since tense consonants are made of doubling the consonant symbol, adding an additional consonant symbol would suffice for representing a syllable with a sense consonant. For example, 끼 may be split to ㄱ기.
The idea of separately enciphering the final consonant above would inspire enciphering a Hangul script by splitting every morphemic block into letters. For example, 한글 (Han-gul) may be split into ㅎㅏㄴㄱㅡㄹ to be enciphered letter by letter.