Text Ads. Advertising requirements
1. In texts of advertisements it is prohibited to use spacing (writing words with spaces between l e t t e r s) and capital letters, except in cases provided for in the rules of the Russian language. In Russian capital letters are generally used at the beginning of sentences, in titles and proper names as well as in abbreviations consisting of capital letters .
2. For some placements in which a text link comprises one word, this word must consist of 10 letters (symbols) maximum.
3. The maximum length of a textual advertisement depends on the project in which it will be placed. The length of the text is specified including spaces and punctuation marks.
Yandex.Direct and special placement
A text block consists of the heading (up to 33 symbols) and the advertising text itself (up to 75 symbols). The number of symbols is specified taking into account spaces and punctuation marks. Please, read the additional requirements (https://yandex.com/legal/direct_adv_rules_ch/) to texts of advertisements and key words in Yandex.Direct.
 Excess formatting for text highlighting purposes – capitalization, underlining, bolding – makes advertising too obtrusive for the user, which reduces the user’s attention to the given advertising place and decreases the advertising effectiveness.
Moreover, according to “netiquette” capitalization of the whole text implies shouting. We do not think you mean to shout at our users.