When CiviCRM moved from SVN to Git, the full history of code changes was not imported into Git. By that time, CiviCRM had existed for more than 5 years and had accumulated a lot of changes, but given the fast pace of the project, it was not particularly relevant to import everything.
The SVN service was kept alive since then, but has had near 0 activity for the past 2-3 years. However, it still occupies resources on our main hosting server (svn.civicrm.osuosl.org, 2 GB ram VPS server), is monitored and requires regular maintenance as any server.
Very occasionally, developers might go back to the SVN archive to check when a feature was introduced (usually by spot-checking branches or tags). The history, while not used often, still provides useful information about the history of the project.
Around 2017, @xurizaemon, who did want to consult the SVN history, but not bother with using SVN, tested a full SVN to Git migration only for archival purposes. Since it seemed like a viable option to get rid of the SVN service, we then did a few test runs and eventually managed to produce a Git repository for historical purposes that had the full history and all the branches (but not tags).
The SVN history only had usernames, but did not have their emails (which Git requires). We tried mapping most of the contributors we could find, usually by looking them up in JIRA. There are a few exceptions where we could not find their email, so we used firstname.lastname@example.org.
2013: SVN to Github migration, SVN was set to read-only