There have been other cases, such as where a mobile app developer has hardcoded an image from someone else's website into their app, then millions of users request it every time they open the app. Or where middlebox manufacturers have hardcoded an IP address to use for NTP.
Sure, having efficient and well-cached infrastructure setup is good, but there's only so much you can do to "reduce server load" where other people in control of widely-deployed software have made choices that causes millions of devices around the world to hammer _you_ specifically.
The people who made those choices don't give a shit, it's not _their_ infrastructure they fucked over. That's why you need to shame them into fixing their botnet and/or block their botnet with extreme prejudice.
Mastodon's link preview service is a botnet, Mastodon knows it, and they refuse to fix it.
Hard to be sympathetic.
This isn't a new issue, nor is it unique to mastodon. Reducing server load for sites like this is a very common exercise for many reasons.