From a technical implementation, we'd probably need some JavaScript that scans page content for external links, and rewrites the URLs to take the user to Special:ExternalLink?url=https://somesite.test?123

This sort javascript of solution could also be a global gadget. Not everyone allows javascript to execute from the wiki context (https://www.mediawiki.org/wiki/No-JavaScript_notes) however.

A more robust solution would be for the parser to emit anchor tags with an href targeting Special:ExternalLink. This could be combined with javascript to bypass the interstitial warning page for users who wanted to opt-out of the warning globally or via some allow list.

One thing to keep in mind while designing and implementing something like this is that it has the potential to accidentally generate toxic user data recording external link usage. Care should be taken to restrict the logging related to this feature to the narrowest viable dataset. I think this would mostly mean attempting to treat the interstitial in the same way that we treat article page views which also have this potential for generating a toxic dataset tracking what has been read by whom.

This is a fundamentally anti-web request and will result in user complaints. And on top of that, I can think of at least one accessibility guideline which this would be counter to: links should act the same whether they're internal or external (vice the other common practice of some websites of always opening external links in a new tab/window). And is definitely unfriendly for anyone parsing our HTML for other reasons.

I can only assume there was another event that caused this task since this is the second similar item showing up in my inbox today, please consider connecting that if it's not already connected.

In T383362#10446414, @Izno wrote:

I can only assume there was another event that caused this task since this is the second similar item showing up in my inbox today, please consider connecting that if it's not already connected.

Yeeeaaah, I’m going to get a gander and point at… this

Should be straightforward technically via the LinkerMakeExternalLink hook, but yes it would raise all kinds of SEO and usability and tool B/C issues, and on Wikimedia sites it would increase the amount of user tracking (currently we don't learn about a user clicking a link, I think it would be hard to avoid that data getting at least into the webrequest table).

In T383362#10446516, @Tgr wrote:

Should be straightforward technically via the LinkerMakeExternalLink hook, but yes it would raise all kinds of SEO and usability and tool B/C issues, and on Wikimedia sites it would increase the amount of user tracking (currently we don't learn about a user clicking a link, I think it would be hard to avoid that data getting at least into the webrequest table).

Yeah I can’t really see this on WMF, at closest would be a setting in core and add to ComConfig? I think this can be made into an extension for third parties to use though.

Please don't do this.

This is going to be incredibly annoying if implemented.

I agree that prompting for every link is massive overkill. But there should be a warning when clicking on a spam-blacklisted link, e.g. in an old revision.