Each version of Pressflow is API-compatible with the same major Drupal version. For example, Pressflow 6 is compatible with all Drupal 6 modules. Pressflow 6 also integrates the SimpleTest system from Drupal 7 and the CDN support patch.
The relevant commits I applied from that repo were:
commit 01cfc83aa6a483654cfed589a50696671014d89b
Author: David Snopek <dsnopek@gmail.com>
Date: Wed Jan 16 11:49:04 2019 -0600
Backport of fixes from SA-CORE-2019-002
commit 8becc15c9d5508216ba4a46a16fef9c2361b63f0
Author: David Snopek <dsnopek@gmail.com>
Date: Wed Jan 16 11:50:34 2019 -0600
Update version number to 6.48
commit e45f482c5fe4bb9127f4aa5cda85f16f6a60a9aa
Author: David Snopek <dsnopek@gmail.com>
Date: Wed Jan 16 14:56:33 2019 -0600
Fix Drush issues from SA-CORE-2019-002
commit 5f5351b88ceed9b790f476888152ce677c99ab70
Author: David Snopek <dsnopek@gmail.com>
Date: Wed Jan 16 15:04:20 2019 -0600
Update version number to 6.49
Applying the changes from D6 LTS using cherry pick from https://github.com/d6lts/drupal.git
The relevant commits I applied from that repo were:
Plus updating the CHANGELOG.txt