The release notes are at https://www.gerritcodereview.com/releases/2.13.md#2.13.8
It includes submodules updates which is the main fix for us as it would have helped me catch problems with submodule problems when we first upgraded to gerrit 2.13.
It also include "Don't truncate long lines in diff screens" fix, though it dosent fix the problem in all cases as I found, but still an improvement to diffs.
"Allow user with "Maintain Server" permission to find all changes."
"Allow to continue reindex despite failures."
"Be more consistent about object ids used in ref operation validation." <-- that one is a major bug fix we want. Though we did not experience it it can happen at any time preventing branches from being created through web ui (maybe also through git too). That bug was exposed when they updated the hooks plugin on master.
Also another fix is "Upgrade JGit to 188.8.131.52703201650-r."
"Among other bug fixes, this version includes a fix for packfile list
inconsistency in memory due to temporary lack of system resources during read, which caused transient "file not found" errors."