Here’s what we get with Dropbox Paper (redacted the URL):
I would have posted the Slack example, but my account is blocked from posting more than one image which seems a little restrictive for feature and bug reports 
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Is it possible that your Slack admin has globally disabled unfurling for the domain? We can do that too, but not for other services like Paper.
Edit: Paper is actually pretty bad, because it fully replaces the URL. At least with Slack, you can read topic titles from the URL.