Reasonably far off topic:

Visa hasn't worked for online purchases for me for a few months, seemingly because of a rogue fraud-detection AI their customer service can't override.

Is there any chance that's just a poorly implemented traditional solution rather than feeding all my data into an LLM?

If by "traditional solution" you mean a bunch of data is fed into creating an ML model and then your individual transaction is fed into that, and it spits out a fraud score, then no, they'd not using LLMs, but at this high a level, what's the difference? If their ML model uses a transformers-based architecture vs not, what difference does it make?