[Attr("my-attr"), Immutable]

Seems a bit disjointed, you could define a property to be immutable without having an AttrAttribute, which won't do anything.

or [Attr("my-attr", AttrOptions.Immutable)]

Allows for multiple option definitions if AttrOptions are defined as flags. But, could also open up the door to trying to do too much with AttrAttribute

or [Attr("my-attr", immutable: true)]