Right now we have FieldType and FieldFormatter plugins. The FieldFormatter can specify which FieldType it can display.
So if I have a FieldType 'float' it can be displayed with DecimalFormatter FieldFormatter which supports 'float' and 'decimal' FieldType plugins.
Now, let's take a look from a different perspective: let's say I have an entity and this entity has a 'price' base field which is basically a decimal(19,4). To display this field I could set it's type to 'decimal' and use the DecimalFormatter to display it. It is functional BUT price is a very specific value/field and needs special handling, like setting a proper currency prefix, maybe current exchange rate needs to be applied and so on. So I have to create a new FieldFormatter plugin that would take care of this.
The issue with this scenario id that the user would still be able to use the DecimalFormatter, which is really bad.
Now it would be better for me to create a new FieldType 'price' and I could then have only one FieldFormatter('price') for it which is great!
Another scenario: my entity has field called 'status'. This is a simple integer. But the value of this field represents a 'state' the entity is in right now. Usually we have something like published or unpublished, but let's take it up a notch and say that we have 10 possible states. Of course I have to create a FieldFormatter for this field which would display the human-readable value of the state instead of the raw value(integer). If I'd create the field via UI I could specify the possible values and display the human-readable value via proper formatter. But since we're talking about base fields we cannot do that and we have to create our own formatter.
But since this field is an 'integer' it can be displayed by multiple FieldFormatters. And this is bad. In this case I would, again, have to create a new FieldType so I can agian have the 1:1 relation with my formatter.
So as you can see, if we need to have only selected formatters available for our base fields we have no other option but to also create a field type which is bad DX. Not to mention that if we would want to allow a (core)formatter that we didn't create for our field type we couldn't since the formatter would not support our field type.
So to avoid this two issues(creating unnecessary field types + inability to have some of the existing formatters available with our field types) I propose the entity base fields could specify which formatters can display the fields themselves. This would be part of the setDisplayOptions() method, let's say under the 'allowed_formatters' key. Of course this should apply to widgets too.
If the 'allowed_formatters' would be set, only listed formatter would be available. If the 'allowed_formatters'would not be set, the usual behavior would apply. If 'allowed_formatters'would be set but none of the listed formatters would exist, the field would not be displayable.