We can support that with a plugin but it won’t be in the toolbar. With the exception of file attachment, the toolbar buttons are only for standard Markdown features.
That’s a major change and haven’t thought much about it.
In general I’m not so keen on non-standard Markdown features because it makes rendering more and more complex and limit the improvements we can make. And the more we add, the more it becomes a hybrid markup that works only in Joplin, which means you can no longer move around your data easily or edit it with other software.
