DB says it was not intentional, and the block seems to have been fixed.

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Deutsche Bahn's booking website (bahn.de) was blocking Linux users due to its bot-detection system flagging the word 'Linux' in the User-Agent string. Testing by heise online confirmed that any browser sending a Linux User-Agent — even on Windows or macOS — triggered error 751. DB responded saying the blocks were unintentional, attributing them to overzealous traffic analysis, and the issue appears to have been resolved, though occasional false positives may still occur.

In a Weird Case, German Deutsche Bahn's Website Was Locking Out Linux Users

The early stages of crawler detection will look like this. I guess websites might start to show Big Tech the middle finger. And for good reason.
Now that Google is breaking the social contract (“we crawl and index your website and you get exposure in the search engine” - by replacing search with a chatbot, as announced at recent I/O, they’re basically becoming leeches), I don’t mind implementing some rules to keep the crawlers out. I don’t need to be indexed when there’s no way for the users to click a link to my website.
I guess we’ll have to get movement to our sites the old-fashioned way: federation and networking. I don’t mind. 🙂