"This network is operating on Wi-Fi channels in use by several other nearby networks."

That message has been around for a couple of years, maybe longer.

That usually means uncoordinated Wi-Fi networks in close proximity are stomping on each other, though it can also mean the local network is somehow corrupted.

Check the Wi-Fi router firmware and update as needed. You can restart the router and see if that clears this, but it will usually return as the other local networks will inevitably shuffle their channel usage back into conflict. Thus if you’re in an area with lots of local networks.

Three other potential workarounds reported elsewhere are to turn off location services, restart the iPhone, re-enable, and check again, and to switch your Wi-Fi router encryption settings (preferably to a higher-grade setting). You can also try to reset network settings on your iPhone, and check again. This if it’s a local settings corruption of some sort. (Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings)

If this is congestion… Migrating off of 2.4 and to 5 GHz—and to 6 GHz, if your Apple gear supports it—can help if the pile-up here is on 2.4 GHz, as can newer Wi-Fi 6 / Wi-Fi 6E / WPA3 gear.

If you have a Mac or PC available, there are tools to scan for local network activity and current channel assignments.

I've never heard of this message before – but it could be that the new version of iOS contained code to detect the condition it warned you about:

"This network is operating on Wi-Fi channels in use by several other nearby networks."

Presumably if everyone is trying to use the same channels, you're dividing up the available bandwidth in more ways, and performance could suffer. So the suggestion to restart the router (and let it choose a new channel) might be a useful one in terms of improving network performance.

I'm not sure why the iPhone was programmed to refuse to connect to the network at all, though.

Your other devices connected to the network in question without complaint – but they may not have had any code to detect the "Wi-Fi channel in use by several networks" condition, or to act on it. You would have seen problems only in terms of connections being spotty, or performance being worse than expected, if your neighbors who were also using those frequencies were doing stuff that involved a lot of network traffic.