The WordPress theme review team, the group that reviews and approves themes included in WordPress' theme directory, recently announced that the Theme Customization API will soon be a requirement for custom theme settings. WordPress added the API in WordPress 3.4 and has encouraged its use for a few years now. The Theme Customization Screen (dubbed the "Theme Customizer") allows admins to edit a theme's settings, color scheme and widgets and see a change preview in real time.
The team sees the requirement as the next step in its ongoing process to transition admins to the Theme Customizer. The requirement was added to the handbook required section. Themes submitted post-announcement fall under the guideline immediately. However, themes submitted prior to the announcement have six months to transition. Accordingly, all custom themes need to utilize the Customizer by Oct. 21. Existing themes will not cease to exist after the deadline; rather, updates made after the addition must utilize the Customizer.
The team recognizes the decision was somewhat controversial and some will balk at the requirement. When WordPress launched the API in 2012, the team hoped for organic adoption, where theme authors would naturally transition to the Customizer. However, that idea did not pan out. The need to force a change arose in order to produce a consistent user experience and standardize the theme review process.
While WordPress has received some backlash from the announcement, it holds to the belief that the decision will benefit all parties in the long run. Once people are comfortable with the Customizer, WordPress believes the process will be more streamlined and the need for users to enter the WordPress admin will be eliminated. For more benefits of the Customizer, check out this blog post by WordPress core developer Nick Halsey.