A couple of weeks ago, Apple began testing out iOS 13.4.5, iPadOS 13.4.5, and a couple of other minor updates. The second developer beta was seeded yesterday, but if you're on the Public Beta Program, you can start testing it out now.
As you'd expect from the version number, there isn't much in the way of new front-facing features in the update. At this point, those are all going to be reserved for the next big major update, which is iOS 14. That's going to be announced at WWDC in June, and developers will get their first taste of the beta after the keynote.
There is one new feature though, which is the ability to share songs, albums, and playlists from Apple Music to Facebook and Instagram. The rest of the update focuses on fixes, stability, and security.
Unfortunately, Apple hasn't provided any proper release notes to detail exactly what's getting fixed. It seems likely at this point that we'll just find out those details when the iOS 13.4.5 and iPadOS 13.4.5 updates are generally available.