I contacted NinjaForms and got the following very helpful response:
I’m so sorry to hear you are experiencing this.
To check the TLS status on your site, a test CURL request is sent from your site to PayPal’s site (https://tlstest.paypal.com) It appears that the site, https://tlstest.paypal.com is down or no longer available which is why you are seeing this false error message.
I have notified our developers of this issue and you should be able to ignore the TLS error warning until we can release an update for the plugin.