I'm not sure it's true exactly - OSS had email as one type of messaging, and IRC as another. The problem is that email lacks instant-ness (and for a long time you couldn't send larger files as attachments), and IRC lacks, or lacked, rich functionality.
Messaging seems to just require more hardware, so the significance of whether the software is OSS or not is reduced.
I'd say IRC is a good example of one of the flaws of FOSS culture - the tendency to get cemented on the first working minimum viable solution, but then become too ossified to ever improve on it. IRC was great for its time, but it doesn't have remotely the minimum set of features the average person expected of a messenger solution 10 years ago. After the initial success of FOSS in chat protocols, almost all of the improvements came from commercial software, and it was too difficult to coordinate introduction of new features across all the implementations
And a lot of this is not even technical, but the cultural issue of scorning anyone asking for those features and claiming those use cases are just for teenagers. Real men just use plain ASCII and no multimedia apparently. Only after its lunch was soundly eaten did we finally get IRCv3, way too late, and still with little support. The reason a lot of younger developers are using Slack and Discord isn't because they're stupid kids, but because their requirements aren't met otherwise, and they're not going to constrain themselves to 90s tech out of stubbornness (to be clear I'm not accusing you of that attitude! I'm commenting on others I've seen many times over the years)
Eh, evolving a protocol is a difficult political issue.
In a business selling software, if you're willing to take some sales loss/mad customers, you can just say in software 2.0, you're going to protocol 2.0.
On the open web/OSS the rest of the world can tell you to screw off... or they can just not upgrade and your software that's a step ahead breaks. Then you also have commercial interests that shove FOSS/1.0 on some device and want to change users to upgrade the firmware so users stay on the old stuff forever.
Commercial software tended to get more features because the software was based on monopolies they had full control of.
Interesting take