Can I get any info on if/when Ignition will support the OPC-UA pubsub specification?
I’m interested in it, but it’s not even on the roadmap right now.
In what way are you interested in using it?
If you want to follow the progress of IA’s OPC server in more detail, you can join the Eclipse Foundation’s Milo project’s mailing list. Ignition v8 has switched to that stack – probably because IA’s own Kevin Herron leads that public project.
Kevin is too modest to point that out… (-:
Ha, thanks. Lately I don’t point it out because I’m embarrassed about the lack of progress towards a 1.0 release 
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Don't sweat it. You're almost (not quite) alone, with a bear of a spec to implement.
At the moment I'm mostly trying to feel out what the options are. I'm new to industry and am working on wrapping my head around the various technologies being used. My team wants to start deploying more lightweight sensor nodes out onto the factory floor and I've been looking into how we might want to go about doing that. A good chuck of the last week has been spent discovering that most people have no idea what OPC-UA really is and then trying to understand it myself.
Am I correct that currently the only non-polling communication supported by Ignition is through the MQTT Cirrus Link modules?
No, my Ethernet/IP Class1 Communications Module supports I/O connections, ad-hoc messaging, and Logix Producer/Consumer Tags. As a target in the base version, and also as an originator in the premium version.
It depends where you want to avoid polling and what devices you're talking to.
Even MQTT solutions will sometimes just be polling at the edge and then reporting by exception over the WAN or whatever link you have back to the central location.
OPC UA subscriptions (that is, between Client and Server) don't technically poll either, but the devices the server is talking to may be need to be polled. It all depends.
For sure. The hard part (or at least one of the hard parts) is understanding what depends on what. I need to sit down with my team and form a clearer picture of what we want for this project and then I'm sure I'll have more questions!