Phil Webb from the Spring Boot team provides an overview of the new features available in Spring Boot 2.3. Click the timestamp to jump to a specific part of the presentation:

Intro:
00:27 Creating a Demo App

Upgrading:
07:24 Version Number Changes
08:29 JUnit Vintage Engine
09:07 Legacy Config Data Mode
10:48 Logback Property Changes and Property Migrator

New Features:
13:51 Java 15 and Startup Logging
15:03 Constructor Binding @ConfigurationProperties Updates
18:11 ApplicationStartup Metrics (Flight Recorder and JSON)
24:49 Changes to Application properties/yaml
30:46 Using 'spring.config.import'
34:46 Importing Files Without an Extension
35:28 Origin Chains
38:34 Config Tree Support (Kubernetes Volume Mounted Secrets)
40:51 Supporting Custom Config Data Import Types
42:57 Docker and Buildpack Updates
46:09 Publishing to a Private Docker Registry

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See also https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.4-Release-Notes