Solution: add http auth to your /wp-admin directory
Edit: Why the downvotes? Lots of non-admin stuff lives inside wp-admin, "add http auth" is terrible advice.
https://censys.io/ipv4?q=%22wp-admin%2Fadmin-ajax.php%22
Hundreds of thousands of sites with "wp-admin/admin-ajax.php" on their index should more than prove this.