As noted here by others: https://lab.civicrm.org/dev/core/-/issues/5421
Short version: public civicrm contribution pages with recaptcha enabled have been failing since about Aug 29 (i.e. 6 days ago).
Unfortunately, this failure doesn't show up, except as non-existent expected contributions.
Updated CiviCRM versions are expected out today or tomorrow.
The official patch is here: https://github.com/civicrm/civicrm-core/pull/31020
In fact, only the first change is important, and it's just removing an extra forward slash in there.
A really quick fix using the unix find, xargs and sed commands is:
$ find wherever-your-civicrm-code-base-lives -path '*ext/recaptcha/lib/recaptcha/recaptchalib.php' | xargs -n1 sed -i "s|request('POST', \$host . '/' . \$path|request('POST', \$host . \$path|"