At my work we use a self hosted Mattermost server.
Shared links in chat are roughly 50/50 between:
- Fun, or informative public websites.
- Internal links, often requiring a login.

For the first a preview is beneficial. It breaks up the wall of text and gives some quick details. These previews are usually unique.

For the latter a preview is useless. Often showing a login-required text. These previews sometimes take a lot of space, and they you always show the same thing (website title, generic description, logo - sometimes large as a preview photo). Examples are: links to internal Gitlab ('sign-in'), a WhereBy channel (generic info about WhereBy).

There is no way for Mattermost to decide when previews are useful or not.

I think it would help if one could control for what websites previews will be shown.

Maybe a setting where one can define a denylist for which no previews, or maybe a very limited one (title, logo), will be shown. I think it would suffice when that denylist is only based on domain names. But more granular support - though hard for many people to define - could perhaps help in some situations.

Or perhaps it would be better to have an option in each preview to mark it as uninteresting. That could hide it (but then you can't unhide it in chat) and any other previews for the same website, or make the preview small (to keep some UI element to revert the choice).