Improvements:

Support for list destructuring and comprehension in for and with expressions. ElixirLS is able to provide completions for destructured list element
Introduction of compile tracers. ElixirLS now builds a databases basing on compile tracers API available since elixir 1.10. References provider has been rewritten to support tracer database
Code action prefixing unused variables with _ Luca Cervello
Complete now proposes not aliased modules and adds required alias Ajay
Custom command running mix clean added. Useful when server hits a compilation error
Custom command returning tests in .exs file
Better handling of Phoenix components Aaron Tinio
Test code lense improvements in umbrella apps 我没有抓狂
Start script improved when $XDG_CONFIG_HOME is not set Sahn Lam
Deprecated symbols are now deprioretized in completions
Improvements to logging
Dialyxir is now vendored. This should avert dependency conflicts
ElixirLS emits more helpful error messages in case of common problems
Automatic builds can now be disabled Hans
Better module name suggested for defprotocol Milo Lee
Improved LSP position handling

Fixes:

Several crashes with untitled: schema URIs fixed
Longstanding bug in dependencies reloading leading to infamous ** (Mix.Error) Can't continue due to errors on dependencies fixed
Fixed crash when formatting a file with syntax errors Steve Cohen
Fixed several crashes in document symbols Steve Cohen

Full Changelog: v0.11.0...v0.12.0