Permalink Manager, like WordPress’s built-in permalinks functions, converts all URL characters to lowercase by default. In general, Google does not care about capitalization, but URLs are case-sensitive, so you should use it consistently.
In general, URL capitalization should not be an issue when it comes to WordPress permalinks. WordPress, in order to maintain consistency, transforms all uppercase letters inside the native slugs by default. As a result, it is best to leave this alone to ensure that all of your permalinks include just lowercase letters. You may, however, use Permalink Manager to permit uppercase letters in your custom permalinks if necessary.
Are the URL addresses case sensitive?
There is a good reason why uppercase slugs are transformed automatically. In general, if the URL does not immediately resolve to a version without caps, and some of your published links alternately contain capital letters, you will have an issue. In other words, case sensitivity in URLs only becomes a problem for SEO when search engines find both lowercase and uppercase variants of your URLs.
Although the tweet cited below is a bit out of date, the comment provided by Google’s John Mueller analyst is still valid.
For additional details on this subject, please see this article on Search Engine Journal or watch the video below.
The inconsistent capitalization of URLs may also make use of the Google Analytics panel more difficult. This is due to the fact that uppercase and lowercase copies of the same page appear independently, each with its own data.
How does WordPress enforce lowercase characters in the slugs?
Although there is no explicit information on how WordPress removes the caps from the slugs, if you are familiar with the WordPress architecture, you should be able to figure it out quite quickly. The strtolower() PHP function is used within the sanitize title_with_dashes() filter, as seen in the screenshot below. This function is linked to the sanitize_title() function, which sanitizes native slugs by removing incorrect characters and accented letters.
How to allow uppercase characters in Permalink Manager permalinks.
Although the lowercase pattern is suggested for SEO purposes, some Permalink Manager customers requested an option to enable uppercase permalinks in WordPress. If you are one of them, please continue reading to learn how to stop the plugin from imposing lowercase letters in the permalinks.
Starting with Permalink Manager 2.0.5.5, you may deactivate the strtolower() function and adjust URL capitalization settings by preventing custom URIs from being converted to lowercase. All you need to do is to paste this line of code to your theme’s functions.php file:
add_filter('permalink_manager_force_lowercase_uris', '__return_false');