I'm unconvinced this is true.

Most of what libraries do technologically is now outsourced to SAAS companies that specialize in the library space (there are at least 2 major one in the US, and I think at least 3 or 4). (encore and bibliocommons are the 2 major ones that I remembered and am pretty sure I've seen at least 2 others).

If everything you are doing is in the cloud, I'd argue that recovering from a ransomware attack is more of a question of if the SAAS company did its job right (which it should be more capable of handling). Of course, that's a reasonable question if they did (similarly why I prefer paying with paypal or google or even (as not so common) amazon third party payment systems than a retailers own internal payment system, yeah they might screw up too, but they invest a lot more into not screwing it up).

The data is probably is somewhat safe in the cloud. I expect what’s infected is a bunch of PC clients locally. The logical thing that could have happened is someone with an admin level of access on the Windows domain opened an attachment or got phished, and that allowed the ransomware to deploy to all the on-prem systems. Unless they have customized the ransomware to do anything special for this particular library software, it may be not all that bad in terms of data at risk. Though it’s possible the human attackers received cloud software credentials, and could have used that to exported patron data to threaten to leak.

There's a good list illustrating the market at https://librarytechnology.org//products/

I've noticed Spydus seems to have a big marketshare in the UK.