The last production release of Houdini included the plug-ins for Unreal versions 4.23 through 4.26, but the latest only had 4.25 and 4.26.

Is that intended or can we get all 4 back soon? 4.23 seems to be missing from the plug-in's Github page too.

We only ship binaries for the latest 2 versions of Unreal, currently 4.26 / 25.
The fact that the prod build had all four versions was a mistake.

You can get the plugin source for 4.23/24 on the git repo, but will have to build the plugin from source.