Deprecated APIs in Drupal 9.0.x core
A key goal of Drupal 9 is to remove deprecated APIs. Tracking the removal of them based on @deprecated and E_USER_DEPRECATED mentions, until the planned beta date on the week of March 2, 2020.
All drupal.org projects by fixability
Ideally projects with no problems found would increase, while every other type of project would decrease as time goes on.
All drupal.org project errors by fixability
Errors in all categories may be decreasing but errors not yet cleanly fixable will likely not decrease for a while.
Top 15 errors across all drupal.org projects
These top errors originally accounted for almost two thirds of all deprecated API use. Fixing them across projects would solve most Drupal 9 compatibility gaps.