Is anyone having issues where the invoices are sending clients to a warning page about the site not being secure? This is incredibly unprofessional for a paid service

Yes - my customers are not able to see their invoices and pay.

The SSL certificate expired at 4 pm for connect.intuit.com...This stupid company has not even considered renewing it....fundamental and basic internet concept from 20 years ago

Yes!

Hi all!

Thanks for reaching out to us about this error message. 

Our engineers are already aware of the error and are already working to renew the SSL certificate. We'll be posting more updates once we receive them.

In the meantime, inform your customers to click the Advanced button and proceed to the site so they can process the payments.

I'd also like to share these articles in case you need more information and reference when receiving online customer payments:

• Take and Process Payments with QuickBooks Payments
• How to Use Payment Links in QuickBooks Online

Let me know if your client was able to process the payment. The Community is always here if you need anything else. 

There really shouldn’t be any transmission of data without a confirmed level of basic encryption especially payment data.

How exactly am I supposed to ask my customers to do that with a straight face?

The renewed SSL cert will have to propagate and browsers will respond without action.

Intuit should never have allowed this to happen, period.  Renewing an SSL cert with DigiCert literally takes minutes.