Babel-users mailing list threads:
- [Babel-users] Higher CPU usage since 1.9.x leads to instability on slow devices
  https://alioth-lists.debian.net/pipermail/babel-users/2020-May/003734.html
- [Babel-users] Removing source-specific for older kernels [was: Higher CPU usage since 1.9.x leads to instability on slow devices]
  https://alioth-lists.debian.net/pipermail/babel-users/2020-June/003742.html

Rules are obsoleted by the removal of the disambiguation code.
See https://alioth-lists.debian.net/pipermail/babel-users/2020-June/003744.html