How do I remove all stale branches from Github?

You can right click on the browser after opening the stale branches and run this in the browser java script console.

To delete all the stale branches. i.e) this script automatically clicks to the next page and proceeds to delete stale branches in the next page.

async function deleteStaleBranches(delay=500) {     
    var stale_branches = document.querySelectorAll('[aria-label="Delete branch"]');
        for (var i = 0; i < stale_branches.length; i++)     {  
          stale_branches.item(i).click();
          await new Promise(r => setTimeout(r, delay));     
        }     
        const next = document.querySelector('[aria-label="Next Page"]');
        if(next) {         
            next.click();
            setTimeout(() => deleteStaleBranches(500), 500);
        } 
}  (() => { deleteStaleBranches(500); })();

To delete the stale branches that is present in the current page only:

async function deleteStaleBranches(delay=500) {     
    var stale_branches = document.querySelectorAll('[aria-label="Delete branch"]');
        for (var i = 0; i < stale_branches.length; i++)     {  
          stale_branches.item(i).click();
          await new Promise(r => setTimeout(r, delay));     
        }     
        
}  (() => { deleteStaleBranches(500); })();