CASS
Cass Information Systems Inc
stock NASDAQ

This is my understanding as well. As someone who works in the industry the FCA have been trying to disuade S+S ISA firms from holding large cash balances for a few years. The purported FCA view is that firms (such as platforms) are not banks and shouldnt act as banks (shadow banking). Makes no sense if the firm is a CASS firm but this seems to be the next step. Interest on cash in a S+S ISA is usually low anyway so it's not usually a good idea. There are legitimate reasons to hold cash in the short term, I.E market uncertainty, so I think there should be a time limit involved such as taxing cash held over 6 months if that is the driver.
TLDR holding cash in a S+S ISA long term is usually it worth it anyway vs inflation.

This is my understanding as well. As someone who works in the industry the FCA have been trying to disuade S+S ISA firms from holding large cash balances for a few years. The purported FCA view is that firms (such as platforms) are not banks and shouldnt act as banks (shadow banking). Makes no sense if the firm is a CASS firm but this seems to be the next step. Interest on cash in a S+S ISA is usually low anyway so it's not usually a good idea. There are legitimate reasons to hold cash in the short term, I.E market uncertainty, so I think there should be a time limit involved such as taxing cash held over 6 months if that is the driver.
TLDR holding cash in a S+S ISA long term is usually it worth it anyway vs inflation.