All messages disappear when SIM card is taken out, very unexpected :-o
All messages disappear when SIM card is taken out, so I assume they're stored only on there. I think this is an unexpected and sub-optimal default, for multiple reasons:
If the messages aren't (also) stored in the home folder, users diligently running backups of their filesystem will simply lose all messages anyway if the SIM card dies, which is bad
It is inconvenient and annoying when switching providers and hence swapping SIM cards. Chatty has no obvious button that I saw to transfer things in that case
It seems just dated and weird, like removing the ethernet cable and suddenly all your offline e-mails disappear. Like not how a modern smartphone should behave, there is enough eMMC/sd card storage after all
Also, there are reasons why one would actually not want to have the SMS stay on the SIM at all, not even in addition to disk storage (although I get that isn't the default), e.g. the modem and therefore by extension the SIM card being way more exposed to government and hacker attacks due to their closed source, obscured, and not very well patched nature. However, this would also require as a first step that the messages are kept on-device and not just on the SIM card.