This is all commentary, nothing really concrete except for Point3 below. Skip if you don't have time.
I have no intention to speak in favor of any specific software, or against. I'm just expressing some concern for the decision process of people who make decisions like this.
WordPress has a "proprietary" HTTP API which plugin developers are encouraged to use. This is a basic wrapper for communications. If cURL is available it simply uses that for transport, but if not it uses other PHP functions. WordPress developers are encouraged to use this API and to avoid coding directly to cURL. But that's a matter of portability, not security.
I believe Joomla does something similar. They use url_fopen or something for package updates but cURL is used if it's available.
cURL isn't much more than just an HTTP transport. It's not inherently any more insecure than any other browser request. If you want security, you need to use HTTPS/SSL just as you would with your browser.
Point 1: These sites that say they don't want cURL may still be using it.
Point 2: cURL manages security very well. For the last
12 years there has only been about one security issue per year, each addressed immediately. I don't know what people do about WP HTTP API security but I know there are security updates all the time - sometimes way too late. I'm sure a lot of people watch the HTTP API for vulnerabilities but somehow it seems a lot of damage gets done in the field before a patch can be put into the field. That's not a criticism. I've been using WP for years. I'm just saying a site can't legitimately say "WP is OK but cURL isn't" - these statements don't make any sense.
Point 3: I'm wondering if MODx has something like the WordPress HTTP API or if it's something to be considered, with full respect for those who have discussed this for WP. In fact, what's wrong with adopting the WP HTTP API and discouraging MODx developers from using cURL directly? And I wonder if that's been discussed in TinyMCE circles which was brought up earlier.
I understand that it's authorities in the French government who are making decisions, and you can't argue with them. I'm just suggesting that if they hear the same thing from multiple developers (that +WP or +Joomla, and -cURL makes no sense) that perhaps they will reconsider the folly, and you might get more support for your projects.