I expect the situation to change over time, OpenBSD has invested a lot in working well with KVM which most providers are relying on now anyway.

OpenBSD also has cloud-init support now[1] which makes interaction with terraform and ansible much simpler.

While the big three do not have officially supported templates, a few providers do, including Exoscale (disclaimer: CTO there).

[1]: https://github.com/exoscale/openbsd-cloud-init