Convert the description field of the 'aggregator_item' entity to a text field

Hm, we don't really have a format for it, as the data is always imported externally?

How do we display it currently ?

$variables['content'] = aggregator_filter_xss($item->getDescription());

Which is

return filter_xss($value, preg_split('/\s+|<|>/', \Drupal::config('aggregator.settings')->get('items.allowed_html'), -1, PREG_SPLIT_NO_EMPTY));

So yes, we could probably define a default format that does more or less the same, no idea what the preg_split() there does, though?

Ah, that's just the splitting of the configuration into an array. So yes, then it would probably be quite nice to convert this into a text field with default format, we'd need to prevent that format from being deleted, though, or convert the setting into a filter selection?

thats a nice idea:)
why dont we use the existing restricted_html text filter though?
the "allowed html tags" setting in aggregator always felt weird.

but whats more important i think is: where we expose the Field UI for items, so this can be made configurable

also. we can get rid of aggregator_teaser_length since, this will be configurable from the field formatter as well

closed #1830068: Change teaser_length to default_summary_length in aggregator? as duplicate

After #2181549: Provide a StringLong field item with schema type 'text' without a 'format' column there would be a string_long field with proper text-blob storage.
Formatter for that field seems will use text module.
The question here how to implement special (locked) text format while aggregator module installed so remove own filtering finctions

Tried to write a quick patch for this, but noticed that aggregator_filter_xss() is also used for the author (only possible to display that in views), and the views integration will need to be updated. Seems a bit strange to also use a text field for that..

also. we can get rid of aggregator_teaser_length since, this will be configurable from the field formatter as well

Unless I've missed something when searching Aggregator's code, the teaser length setting can already be removed. I'm pretty sure it doesn't do anything in D8. See #2283877: Remove or use Aggregator's teaser_length setting.

aggregator_filter_xss() is gone but I don't think this can be done in 8.0.x. *Maybe* 8.1.x if we write an update function and don't consider the field type/storage an API, but maybe we have to.

This issue seems to be covering two things

1) Use a formatter instead of extra field info. This is part of #2353867: [META] Expose Title and other base fields in Manage Display. The first step is included in #2993642: Mechanism to disable preprocessing of base fields in taxonomy and aggregator entity types so they can be configured via the field UI - that removes the special case processing so that a module could enable a formatter.

2) Converting the field from string to text would be challenging for BC. However do we really need that? It could work well if it remains as string with the aggregator_xss formatter. Then it doesn't need any complicated code to ensure a particular text format always exists.

The aggregator module has been removed from Core in 10.0.x-dev and now lives on as a contrib module. Issues in the Core queue about the aggregator module, like this one, have been moved to the contrib module queue.

+1 for splitting this into two issues per the comment from @AdamPS