One of MediaWiki's greatest strengths is the diverse extension ecosystem. But we (MediaWiki core developers) can often make it hard for extension developers to keep up with constant breaking changes. We do not have a standard deprecation policy, leading to widely inconsistent practices depending on who writes and/or reviews the patch. Some reviewers expect you to fix all instances in Gerrit before merging a core deprecation patch, others will be fine with fixing none.
T114384: Standardise procedures for deprecating public-facing code
T149727: Deprecated MediaWiki extension API functions should be moved to a compatibility library instead of being dropped entirely
T115341: Create a standard timetable for deprecating public-facing code across all WMF projects