Is it safe to delete the log4j files contained in the upgrade-backup folder created after a Portal for ArcGIS upgrade?

Answer

Yes, it is safe for those files to be deleted.

When Portal for ArcGIS is upgraded, a folder named upgrade-backup is created in the arcgisportal folder:

Default location on Windows:

C:\arcgisportal\upgrade-backup

Default location on Linux:

<install path>/arcgis/portal/usr/arcgisportal/upgrade-backup

The arcgisportal folder may be in a custom location depending on the installation configuration.

This directory contains version-specific folders with backed-up contents.  From Section 2 of the Perform an upgrade of your portal deployment documentation, it is recommended to move that folder to another location to save disk space.

The upgrade-backup folder contains multiple log4j files under /update-backup/<release>/dsdata/elasticsearch_#.#.#/lib.  It is safe for those to be deleted.