Classic Orthography
In the simplified orthography, diacritic letters are automatically included - words are written with these letters which were resolved by Levenshtein algoritm. If a diacritic letter is popular in Slavic words, and is prevalent, the word containing it is automatically recognized by algorithm as the best (closest) to other Slavic languages.
However this situation is rare. A good example is verb "děla~". The letter "ě" occurs as combination of soft sign and e. The equivalent "ie" combination is occuring less frequent that "ье". In other example of words "viera" and "věra", the version with "ie" wins over "ě" as "ie" is more popular version of orthography.
In the classic version of orthography, it is forced to write ancient Slavic letters, ѣ = ě (yat), ѧ = æ or ę (nasal little jus), and ѫ = ų (nasal big yus), wherever there is a trace that such orthography existed, even if the algorithm doesn't resolve so, because the letter has vanished already in Slavic languages.
To investigate where these letters existed a review of languages Polish (nasal vowels), Old-Ruthenian (yat) and Old Church Slavonic (little/big yus)
must be conducted. If diacritic letter exists in a word, such orthography can be used in "classic" writing system.
Here are some words in classic orthography versions:
jæzyk/język/јѧзык (language), мѫж/mųž (man), пѫт/pųt (path), мѧсо/mæso/męso (meat).
Whether this is beneficial for the language or not, this is to be assesed by every speaker and writer. Definitely both forms will have fans and opponents. However a well educated Slav should know the basics of classic orthography, not to be surprized by texts with "a lot of strange characters".
Yet another letters exist namely ъ (hard sign / schwa) which is latinized as ″ in ISO9 standard or written as o in Russian. To distinguish it from regular "o" a letter "ȯ" (vanishing o) can be used in latin transliteration of words like къгда/k″gda/kȯgda (when).
And the last interesting phenomenon, letter å which occurs in words like врåна/vråna (crow) or крåва/kråva (cow). This is a trace of ancient o/a letter which in different Slavic languages were written either as o or a (Czech vrána, Polish wrona). Due to lack of proper latin letter, the orthography diverged. Now we can consolidate it back with the letter å.
It is hard to list them all, but good to know what these letters mean. They are the sentiment towards classical times, where Slavic language had nasal vowels and other sounds not able to be rendered by latin script.
Should we just forget them after thousand year history or preserve for future generations? You will have to answer this question by yourself.
Triod cvĕtnaă (1491)