Custom ASCII character mappings By default, if we use cyrillic in the names of entries, the slug is also created in сyrillic. This is not good. And here we need transliteration such as б → b, д → d, etc.
By the way, if we create for example a new section in сyrillic, the handle, Entry URI Format and Template is created almost correctly, the exception is the characters 'Ь' and 'Ъ' - they are replaced by a space.

Ok, in general.php add
'limitAutoSlugsToAscii' =&gt; true

to replace characters according to ASCII character mapping. Now slug in entries should be created correctly, but again a new problem with the characters 'Ь' and 'Ъ', they are not converted.

in general.php add
'customAsciiCharMappings' =&gt; [
    ' ' =&gt; ['ь', 'Ь', 'ъ', 'Ъ'],
],

and we get the same result as when creating a new section - space.
Tried this variant:
'customAsciiCharMappings' =&gt; [
    '' =&gt; ['ь', 'Ь', 'ъ', 'Ъ'],
],

does not work
In the correct convertation there should be no spaces or replacement, 'Ь' and 'Ъ' should be removed (Мальчик → malchik). How to remove these spaces?
Ignoring customAsciiCharMappings for the moment, Craft uses Stringy for this type of thing.

The have some special logic for German and Bulgarian character replacements and it looks like those particular characters will only get replaced if the locale of the site you're currently editing is bg: https://github.com/danielstjules/Stringy/blob/master/src/Stringy.php#L1874-L1877