I suppose this is a decent workaround for certain things (i've used it in sqlite before), the main kind of index i'm using with postgres jsonb looks something like this

    create index on my_table(document ->> 'some_key') where (document ? 'some_key' AND document ->> 'some_key' IS NOT NULL);

you can use generated columns to get around the first part of the index, but you can't have the WHERE part of the index in mysql as far as I am aware (but it has been a very long time since I've worked with it so I'm prepared to be wrong).