Messages are end-to-end encrypted in iMessage, meaning Apple cannot read the message contents. This solution would require Apple to bypass that encryption for URLs (which are often privacy-sensitive).
A good approach would be for the sender to fetch the URL and embed the preview as metadata along with the message. The only downside is that the sender could spoof the preview, but I think that's an acceptable trade-off here (not much of a phishing vector when you end up loading the original site once you open the link anyway).
No need to do in transit. I mean iMessage could simply proxy all http/https requests post decryption in iMessage, pre-request.
At the end of the day this privacy trade off (apple gets your browsing info) is probably more secure than an embedded webview that could potentially be exploited and is auto-loaded. Similar to how Chrome alerts of malicious sites...I see this as a long term larger attack vector than privacy leakage.
The URL being disclosed to Apple was what I was getting at, which would happen with any approach that involves Apple performing the request on behalf of the user. I don't think the trade-off you're describing is necessary given that the sender could prepare the preview.
A good approach would be for the sender to fetch the URL and embed the preview as metadata along with the message. The only downside is that the sender could spoof the preview, but I think that's an acceptable trade-off here (not much of a phishing vector when you end up loading the original site once you open the link anyway).