I'm not defending Cisco, but just observing that not only is OpenVPN not their product, it is a competing technology. They added it in firmware version 126.96.36.199 on 7/20/2016, but the release notes don't say why. My personal suspicion is that Cisco did not keep up with the SSL VPN, because it was not compatible with newer browsers, and OpenVPN may have been a work-around offering as a courtesy. We use it and have found it very useful; it's certainly easier than setting up an OpenVPN server on a separate box.
I do agree, however, that if Cisco put it out there, they should either maintain it for functionality and security or remove it altogether. I'm not holding my breath and in preparation of the April change, I migrated our setup to OpenVPN running on a server.
I completely agree with you and fully appreciate that every vendor has the right to determine which services, standards and protocols they want to include in their products and at what price. However, when products ship with a capability, you would expect a vendor like Cisco to continue supporting it within reason for a reasonable lifetime of the product.
If this was DLink, I would have different expectations.