Your website is vulnerable to Man In The Middle attacks on subdomains because the
SECURE_HSTS_INCLUDE_SUBDOMAINS setting is missing - so a hacker can intercept and change requests performed over HTTP.
If your website should be accessed via HTTPS then consider enabling HSTS and setting
SECURE_HSTS_INCLUDE_SUBDOMAINS to protect subdomains. This will make the browser block HTTP requests to your website and instead use HTTPS.
HSTS can be thought of like "client side HTTP to HTTPS redirect": the browser will turn HTTP to HTTPS requests.
If your website is using HTTPS, then it is likely that your website redirects HTTP to HTTPS (if not, it should). However, a Man In The Middle attack can intercept the initial HTTP request and instead serve the user a malicious webpage that looks like your website.
SECURE_HSTS_INCLUDE_SUBDOMAINS reduces the change of that because the brower will simply not do HTTP requests to your website.
Note it's safe to set
SECURE_HSTS_INCLUDE_SUBDOMAINS in your local dev env, as the header is only set when accessing the website via HTTPS, not HTTP.
If our GitHub code review bot spots this issue in your pull request it gives this advice: