Even small banks. Basically every bank that was around 20 or 30 years ago started with mainframes (or very large minis) and still uses them. I'm not in that business, but I've seen the server rooms, and I haven't heard of a bank switching over to a rack of PCs, yet. There might be, but that would still hundreds, perhaps a thousand banks in Europe alone running on large-ish iron.
There might be less mainframes in total, though. The bank where my parents worked had 4 at different sites, but now runs on a single one, AFAIK.
> I haven't heard of a bank switching over to a rack of PCs, yet.
Right. The main limitation is the core software platform. This is an extraordinarily complex system that requires expensive regulatory sign-off/audit/etc.
There are some vendors in the space who are trying to do a new bank + new core. Even if someone had an awesome x86 clustered core system with 100% regulatory approval, any existing mid-tier bank converting to it would be an ~8 figure ordeal.
But the OS, database, network definition and workflow languages are still proprietary. Which goes back to the original question: where is that knowledge store/discussed?
There might be less mainframes in total, though. The bank where my parents worked had 4 at different sites, but now runs on a single one, AFAIK.