If you have any improvements you want to contribute back, the best is to use git to get a local copy of the source code, apply the fix and put out a Pull request so your improvements can be integrated quickly. Otherwise you can also create new Tickets in the issue tracker: https://github.com/geonetwork/core-geonetwork/issues
Thanks and congratulations to the all community members and developers that have been putting efforts and energy in this release!
IMPORTANT: If you have a custom metadata schema, please check the information in https://github.com/geonetwork/core-geonetwork/pull/2793, as will require a change in schema-ident.xml to get them working in version 3.4.3. The metadata schemas packaged in GeoNetwork are already updated.