A custom recovery such as TWRP allows you to install custom ROMs and root your devices more easily. Unfortunately, the development of the software for the Pixel 4 and 3 on Android 10 has hit major roadblocks. Some changes in Google's implementation of the recovery and personal circumstances of the main contributor to TWRP could mean that it'll take a while until it's available.
On the technical side, Google moved a lot of AOSP recovery components into subfolders, so merging the latest changes into TWRP is going to be a more lengthy affair this time. The company also introduced some completely new steps to the boot process. The ramdisk TWRP normally uses for recovery is now additionally handling part of the regular boot, so it remains to be seen what the team can do to work around this problem. Said ramdisk also switched to dynamically linked libraries, which could introduce further challenges, as TWRP used to rely on separate linked libraries.
Even once TWRP is compiled with the changes from Android 10, there's more work. The new ramdisk places executables and libraries inside a system folder, which TWRP hasn't touched in the past. Mounting the system partition could get tricky when the team leaves things as they were on Android 9 and below.
Meanwhile, Dees_Troy, the developer usually in charge of merges for new Android versions, has a wife pregnant with their fourth child, so you can imagine he's quite busy with non-tech stuff in his spare time. He invites you to "feel free to download the TWRP source code and make the needed changes yourself," otherwise, patience is the key to a custom recovery for Android 10.