About moving built-in packages to ELPA

[xenodasein]

Are there plans regarding to this?

Some packages require active development and some don't; two extremes that come to mind are Tetris and CEDET. The former is at this point a signature Emacs feature and lightweight enough to be a lisp package so it makes sense to keep it built-in. The latter would need to constantly keep up with the developments of the programming languages it supports; making it optional would free up developer time. (Especially taking into account that lsp based alternatives seem to be overwhelmingly more popular as per the survey.) If it is considered a core feature of Emacs, such heavy packages would benefit greatly from migration to C code.