There is nothing technically stop the carrier putting more memory on the SIM card.
The big iffy is that manufacturers were compromised (by the US afaik) in 2015 and all cards from then or earlier should be considered compromised.
Few carriers encrypted their OTA-keys before then. I know that some carriers did not send encrypted keys to manufacturers still in 2018.
Today, it's mostly Java Card implementations, meaning that the ISA and smartphone OS/runtime is abstracted away in any case.