Take a skim through the link for full details (especially the breaking changes), but I have included some parts that I thought were important:

This release has been over two years in the making, so we're really glad to finally get it out to you. The long cycle does mean quite an extensive changelog however, with well over 1100 pull requests merged into our master branch since 10.8.0 first dropped back in 2022.

General

• We now support "trickplay" a.k.a. live video scrubbing. When scrubbing through a video with this enabled, you will be able to see a live preview of the video at that timestamp. Note that this requires explicit client support, which may require some time to become available depending on your client.

• [...]

• We now support AVIF and WEBP images for Pictures libraries.

• Tags are now accounted for during searches, allowing one to search by tag.

• We now support multiple simultaneous subtitle tracks (maximum of two, a primary and secondary) in the web player.

• We've revamped the administrative dashboard UI to help improve usability and ease of finding options.

API & Security

• All API endpoints now return proper return codes, ensuring that API endpoint results can be reliably interpreted without additional parsing.

• Parental ratings are significantly improved, with better enforcement, inheritable ratings, and more.

• LiveTV and Collection permissions are now discrete and configurable per-user.

• The EasyPassword (PIN) feature has been removed as this was a big security risk especially for administrator accounts; QuickConnect login is still supported however.

• User permission handling has been unified and numerous bugs fixed, ensuring a more secure server from untrusted users.

Core Server & Networking

• [...]
• The server now supports in-process restarting, and removes the old hacky restart.sh method. This should make things like installing plugins much more robust and ensure a consistent restart experience regardless of platform or install method.
• [...]
• The backend SQLite database now supports connection pooling, which should improve performance for database operations.
• [...]

Also sections on Packaging, Transcoding & FFmpeg improvements/support, Scanning, Library & Playlist Management, and Casting

The Next Version

With our continuous integrations improvements outlined previously, we're quite confident that this will be our last "very long" release cycle. Our plan is for the next major version (10.10.0) to be released at most 6 months from now, some time in October. We hope this increased cadence will help alleviate the problems with large releases such as a very long time-to-stable for new features, translations, etc. and help lower the number of major bugs at each major release, streamlining the upgrade process. But this needs everyone's help. Back in October 2023, we made a call for developers, and we've gotten a lot of interest, but this is not a one-and-done event. We need contributions now more than ever, especially around the web frontend to help implement our planned design changes. If this interests you, please reach out and we can help get you set up.