I got this question recently in meet of link builders held in our city. Either we should use capital letters or small letters or the first character of each word with a capital letter in the keyword phrase. I found there is some clue in it. I think search engines understand the links differently to a small extent with the small or capital letters. Let us see the example of how the “search engine optimization” can be used in anchor text

Search Engine Optimization
search engine optimization
Search engine optimization
SEARCH ENGINE OPTIMIZATION

I search all the above keywords written in different styles and found there is a minor change in the number of results Google presents. With the numbers of different results in Google search for each keyword, it is true that the search engine takes them separately to some small extent. “SEARCH ENGINE OPTIMIZATION” we can consider somebody is strongly recommending it. “Search Engine Optimization” can be understood as a specially mentioned keyword by search engines

If the anchor text is in small letters it is natural and organic in my view. If you look for contextual links you will find all links are within the text and are on small letter written keywords

In my opinion, we should use the mix of all to make our link building strategy or campaign organic and natural.