> If in fact you "want to allow my customers to send me encrypted data via an untrusted channel" in 2019 you should just set up a secure Web Server, with a Fully Qualified Domain Name your customers know (e.g. write it on business cards, or the sides of vans, or a huge building) with a form on it, where they can submit things.

That assumes the customer's browser is trusted, and not configured to silently allow all encrypted connections to be intercepted, decrypted and potentially modified by someone else in the network, which is unfortunately still too common.