The use of "varchar_ascii" in (some) fields is problematic

That is Drupal core defining the COLLATION, not Drupal Commerce.

// Note we check for the "type" key here. "mysql_type" is VARCHAR:
      elseif (isset($spec['type']) && $spec['type'] == 'varchar_ascii') {
        $sql .= ' COLLATE ascii_general_ci';
      }

See the Mysql schema implementation from Drupal core: https://git.drupalcode.org/project/drupal/-/blob/11.x/core/modules/mysql...

I disagree.
The type "varchar_ascii" is explicitly set in some commerce schema field definitions. It could also be defined as "varchar", which would default to the collation used by the db/table and "fix" the issue.

Or do you mean that Drupal core should be responsible for correctly handling these ascii mismatch errors?
If so, is there a core issue that you can refer me to?

Ok sorry you're right, replied too quickly here... We indeed explicitly specify "varchar_ascii" for certain fields... I can't recall why TBH... I've never experienced this issue previously though...

Also FYI, any new bug report / issue should be created against 8.x-2.x.

A special varchar_ascii type is also available for limiting machine name field to US ASCII characters.

Ok so that might be why then... Only fields Commerce defines using the "varchar_ascii" type are for fields holding values that are "identifiers" / "machine names"...

Ok, gave a closer look also at your error message.
The "problematic" field here is a standard core Entity reference field.

Core explicitly defines the "varchar_ascii" type for holding the target_id of the entity type the field is targeting. This is done for config entity types (see the else here in EntityReferenceItem::schema()):

    if ($target_type_info->entityClassImplements(FieldableEntityInterface::class) && $properties->getDataType() === 'integer') {
      $columns = [
        'target_id' => [
          'description' => 'The ID of the target entity.',
          'type' => 'int',
          'unsigned' => TRUE,
        ],
      ];
    }
    else {
      $columns = [
        'target_id' => [
          'description' => 'The ID of the target entity.',
          'type' => 'varchar_ascii',
          // If the target entities act as bundles for another entity type,
          // their IDs should not exceed the maximum length for bundles.
          'length' => $target_type_info->getBundleOf() ? EntityTypeInterface::BUNDLE_MAX_LENGTH : 255,
        ],
      ];
    }

So not a lot we can do here... What we did elsewhere in Commerce just follows this pattern.

See the related issues from 📌 Throw an understandable exception when there is an attempt to load config entities with disallowed characters Needs work . There is likely one of the issues matching your bug report.

Marking this as won't fix, as for this particular issue reported, any other entity reference field would trigger the same issue elsewhere in Drupal.