Formerly, btsync but now acquired by Resilio, rslsync is perhaps the only accessible online tool that uses torrents for file synchronization. However, rslsync is not free, but for at least two decades, there have been no shady practices such as making previously purchased licenses unusable as an excuse for bumping the version, such that rslsync seems like good business.

The replace_response module is needed and has to be installed using xcaddy. With the replace_response module installed, the following caddy configuration will reverse proxy the URL http://hostname.tld/sync to the Resilio Sync web-interface residing on the local machine and port 8888 (127.0.0.1:8888):

{
        order replace after encode
}
:80 {
        redir /sync /sync/
        handle_path /sync/* {
                rewrite * /gui{uri}
                reverse_proxy 127.0.0.1:8888 {
                        header_up Host 127.0.0.1:8888
                        header_up X-Forwarded-Host {hostport}
                        header_up Accept-Encoding identity
                }
                replace {
                        "/gui/token.html" "/sync/token.html"
                        "/gui" "/sync"
                }

        }
}