Before
Any text format that uses a contrib filter gets a filter_null filter inserted automatically (the only exception appears to be the Markdown module + filter, thanks to #2711053: Migration from D7 broken in 8.1.x). Any time filter_null is used, all body fields using that text format show up as empty.
Even if the filter is purely cosmetic: typographical enhancements, URL transformations, tokens getting expanded — those things not working automatically after a migration is acceptable. It's easy to install add a filter to a text format after the migration. That's the whole point of the filter system: filters are non-destructive. Which means the user's original input data is never modified, and any fancy transformations happen at render time only.
This looks broken and is scary: it seems as if all content was lost during the migration. It makes it seem as if content was destroyed.
After
Maintain in Drupal 8 core's \Drupal\filter\Plugin\migrate\process\FilterID filter types for the most commonly installed contributed filters.
If a filter does not have a Drupal 8 equivalent and it is a transformation-only filter:
- drop that filter from the text format
- warn the user
Notes
The top 300 editor/filter modules were audited, or the first 6 pages out of a total of 14 pages. The pages were sorted by most installed first. Module 300 had only 89 sites using it. That's less than 0.013%.
Which means we're firmly in the 99.9% territory of D7 sites benefiting from the metadata incorporated into Drupal core.
