For convenience of demonstration and development, I tend to store client sites on my demo web server. However, I recently realized that all these sites were being indexed, some even pushing real sites out of search results. Is there a way to fully prevent indexing for a particular domain, including its subdomains, without constantly relying on robots.txt? The issue with putting it in a folder is the potential for forgetfulness or failure to delete it from the client's site.
 Are there any alternative solutions to address this problem?

Is it really possible to forget to change or delete the robots.txt file, as you would not forget to update database connection settings or add counters? Is it worth exploring alternative options such as using meta tags when building projects for greater convenience in site management?

In hosting, the subdomain is not treated as a separate site or folder in the main domain's folder structure. For instance, when "http://test.site.com/" is created, it will appear under "site.com" as a test folder. Therefore, robots.txt file is used to regulate search engine indexing. The common practice is to disallow indexing for the test folder through the following directive:

User-agent: *

Disallow: /test

However, this approach might have some nuances. One possible solution is to create a separate robots.txt file and place it in the subdomain's folder with the same disallow directive.

Yes, there are several ways to prevent robots from crawling and indexing your websites. Robots.txt is the usual way, but I understand you're looking for alternatives that might be more foolproof.

Here are a few methods to consider:

HTTP Response Headers: You can use the X-Robots-Tag in the HTTP header to control indexing for a whole site, including its subdomains. You can set it to 'noindex' to prevent all pages from being indexed. This method is useful when you want to prevent indexing of specific file types which robots.txt doesn't support. You can implement this at your web server level (like Apache or Nginx). Here's an example of how to implement it on Apache in your .htaccess file:

<IfModule mod_headers.c>
  Header set X-Robots-Tag "noindex, nofollow"
</IfModule>
And for Nginx in your server block configuration:

add_header X-Robots-Tag "noindex, nofollow";
Meta Tags: You can instruct search engine bots not to index your pages by using meta tags. You need to add a 'robots' meta tag to every page with the content 'noindex'. However, this option is impractical for large sites since you need to update each page individually.

Password Protection: Any site or part of a site that is behind a login (even a simple .htaccess one) will not be indexed by search engines since they will not be able to bypass your login process. By password-protecting your entire site or demo areas, you're ensuring robots won't be able to access it.

Disallowing via Search Console: For Google, specifically, you could use the URL parameters tool in Search Console. This method wouldn't necessarily apply to all search engines, but it would work for one of the primary ones.

Using a Development Domain: Create a specific domain or use a universal naming convention for your development servers that you can uniformly instruct search engines to ignore. This, however, will require the usage of robots.txt or X-Robots-Tag, but can be implemented universally.

Server-side Scripts: Use a server-side script (like in PHP, ASP.NET etc.) that dynamically adds a noindex tag depending on whether the website is live or not. Set an environmental variable on your development server that the script checks to decide whether to add the noindex tag.

Canonical URLs: You can set a rel="canonical" link element on every page of the development site, pointing to the corresponding page on the live site. So, even if the development site does get indexed, all the ranking signals should be transferred to the live version of the site.

Use Nofollow Links: All the links that lead to the development or demo site should be set as 'nofollow'. This means that while search engines can still find your site, they will not index it.

Using a Test Server without DNS: If you are primarily using the server for testing, consider not assigning a domain at all. Allow only IP based access or local DNS.

However, as per your requirement, the HTTP Response Headers method seems to be most effective in your case. It requires less manual intervention and there's a lower risk of missing anything, as it can be managed at a web server level. Nonetheless, remember that there is no fail-safe method for preventing all search engine bots from indexing a website or webpage, as some bots do not follow the rules laid down in robots.txt, meta tags, or HTTP headers.