This is a follow up issue to https://www.drupal.org/node/2504759 →
migrate-upgrade is still missing the --legacy-private-path command-line option:
--legacy-private-path - Base from which to fetch private files, if anyway. Obviously these can't be scraped, they'll need to be provided locally (not this command's problem).
This is important to migrate private files similarly to the "Private file directory" provided by "Migrate Drupal UI".
Needs work
3.0
Code
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Sorry to not comment on this for so long -- I have applied this patch and it does make the option available and apparently accepts it and uses the variable - but I continue to have problems with the path being formed incorrectly as discussed in https://www.drupal.org/project/drupal/issues/3123350 → - I have also applied this patch and the path is still being formed incorrectly by concatenating the public and private paths together.