There is no “undelete” bulk action that I know of, so you’ll need to write a script for this and use the Discourse API. Everything you do in Discourse you can do through the API, check this topic to learn how How to reverse engineer the Discourse API
As instructed in the topic, if you check your network tab (in your browser’s dev tools), you’ll see that whenever the “reverse delete” button is clicked, there is a PUT call made to /posts/<post-id>/recover
So, if you have a list of IDs of the posts you want to “undelete” then you could do so by making successive calls to that end point.