[META] Beta blocking upgrade path bugs

Problem/Motivation

For people to be able to update to Drupal 9, we need to resolve two categories of upgrade path bug:

1. Upgrade path bugs preventing people from updating from Drupal <= 8.7.x to Drupal 8.8

2. Upgrade path bugs preventing people from updating from Drupal 8.8 or 8.9 to Drupal 9 itself.

There will be a handful of upgrade path bugs which while critical, don't actually need to block a Drupal 9 release. This would mainly be issues where configuration is updated incorrectly, but it doesn't block the upgrade itself, prevent site operation, and where the data can still be repaired in Drupal 9. See the two views issues listed under 'should have' below for examples.

Generally we should assume that an upgrade path bug does block the release, then move it to should-haves if we determine that it doesn't.

Proposed resolution

Ensure that all upgrade path bugs have been triaged, and are listed under either must-have or should have.

Remaining tasks

Must haves

update_fix_compatibility() puts sites into unrecoverable state #2917600: update_fix_compatibility() puts sites into unrecoverable state

system_update_8804() fails if any path alias is created before it runs #3098718: system_update_8804() fails if any path alias is created before it runs

SqlFieldableEntityTypeListenerTrait doesn't take additional entity base fields into account
#3056539: Updating an entity type from non-revisionable to revisionable fails if it has non-revisionable fields stored in dedicated tables

taxonomy_post_update_make_taxonomy_term_revisionable() fails when terms have no default translation #3056543: taxonomy_post_update_make_taxonomy_term_revisionable() and the menu link content equivalent fail when entities have no default translation

Update from 8.6.15 to 8.7 fails due to corrupt "menu_link_content" entity data #3052318: Update from 8.6 to 8.7 fails due to corrupt menu_link_content or taxonomy_term entity data

Manipulating the revision metadata keys before running the BC layer breaks the BC layer: #3098427: Manipulating the revision metadata keys before running the BC layer breaks the BC layer (for 8.x) and #3099789: Remove the BC layer for revision metadata keys (for 9.x).

Should have

views_update_8500() inlines configuration changes instead of this being done on config save for bc #2989745: views_update_8500() inlines configuration changes instead of this being done on config save for bc

dblog_update_8600() breaks views.view.watchdog config entity, leading to PHP warning on admin/reports/dblog and configuration import errors #2998103: dblog_update_8600() breaks views.view.watchdog config entity, leading to PHP warning on admin/reports/dblog and configuration import errors

User interface changes

API changes

Data model changes

Release notes snippet