Problem/Motivation
After #3009014: Convert path alias migrations to use entity:path_alias destination, the d6_url_alias_language is no longer used (nor needed), so it needs to be deprecated, per @mikelutz in comment #24 in that issue.
Proposed resolution
Do it :)
Remaining tasks
Code, review, etc.
User interface changes
Nope.
API changes
Nope.
Data model changes
Nope.
Release notes snippet
Nope.