Our codebase is around 20 years old and was in CVS, then SVN, then git. Then several years after git, a new git repo without any history due to poor use of the first git repo (someone added binaries, bloated the repo to GBs instead of maybe 200-300MB, which made git export horridly slow).
In all the steps we preserved the commit history, except for the final git->git. However also when we moved from SVN to git we kept the old svn server running as a historical archive for several years, as we didn’t carry across all projects (some were already EOL’d years ago).
During that time I looked at it maybe twice, and ultimately we decommissioned it.
Likewise with the new/old git repos, we still have the old git repo if we need the history.
One final thought: git blame was nice, until someone reformatted the entire codebase and committed it back in (we’ve since adopted better git workflow and code review practices!)