Yes, second parameter for load_plugin_textdomain() is a deprecated parameter. It makes no sense to keep carrying that on. As I said here: “Thoughts on breaking changes and a native CP pre-Gutenberg plugins ecosystem”, the time to break from WordPress with a CP native Plugin (and themes) directory is now.
In the process of doing that, some subtle changes will have to be applied to those plugins, but nothing crazy. And removing a useless variable that has been sitting there for years is something that takes less than 15 seconds to do. It takes longer to write your message than to fork a pre-Gutenberg plugin and apply the two or three subtle changes… 
- Also, I offered to do that myself, for plugins requested by the community: “What are some of the most important open source plugins in the pre-Gutenberg WP ecosystem? I want to try to keep them compatible with the CP Next version I am working on” (with the idea that they can get into the potential new Plugin Directory)
I’m not removing anything crazy from core. And actually, I’ve already removed everything I wanted to. The next step is all about separating core stuff into plugins.