The git client had protocol v2 introduced in 2.18. It has then been further improved and 2.26 turns it on by default.
Debian has the following versions:
Jessie is EOL and we only have a few apps and CI containers still using it.
We should probably avoid Debian backports since they disappear before the distribution is EOL. Then it is straightforward to have a version kept in a component to prevent this.
The big question is what was still broken from 2.20 to 2.26. But maybe 2.20 is fine.
We have the list of versions in use on servers and images at https://debmonitor.wikimedia.org/packages/git