Static methods are widely used throughout the MediaWiki code base. This introduces procedural code, global state and makes it quite a challenge to do good unit tests of the MediaWiki code. It is difficult to isolate pieces of code and test each piece, each class, each method and is not possible to mock the static methods of another class.
One of the most egregious examples is in the Linker class, whose static methods are used nearly 500 times throughout MediaWiki. Especially heavy use occurs in special pages, ChangesList / Enhanced Changes, Difference Engine, etc., making them difficult to test fully.
MediaWiki should move away from using static functions, and a first step or trial of this would be to replace the Linker utility class with service objects. Then other classes using Linker can mock out those methods and isolate their own methods for testing.
Another issue with Linker is that it mixes many different functionalities that are not necessarily directly about "linking". Linker includes code that handles formatting comments, such as in recent changes pages. This should be separated into distinct value and service class (Comment/Summary + formatters). Linker also contains code for formatting the table of contents in pages. This could be split into its own formatter class.
Any refactoring of the Linker class needs to keep performance in mind and how the code interacts with the LinkCache. Methods such as Linker::link() can be quite inefficient, especially if called numerous times. (e.g. on watchlist) In refactoring this code, may also be able to look into how to improve that
More details: mw:Requests_for_comment/Linker_refactor