I saw this from bbPress sites. Even if your theme doesn’t link /reply/ anywhere Google will still find it sometimes from your sitemap.xml
There is no fix after many years, but you can noindex using Nginx:
location ~ ^(.*)/reply/$ { try_files $uri $uri/ /index.php?$args; set $robots "noindex, nofollow, nosnippet, noarchive"; }
SlickStack includes this by default
https://github.com/littlebizzy/slickstack/blob/master/modules/nginx/sites/production.txt
bbPress gang were talking about this several years ago with no fix:
Reply pages & SEO issues
that thread is like 10 years old and they never fixed this problem?
Enter the destination URL
Or link to existing content