It would be prudent to assume they would backdate reporting from (at the minimum) when they signed up to
CRS, but they would probably do so even before that. A lot of people are getting their panties wet, somewhat unnecessarily I feel, over CRS and AEOI. Just because a country has signed up (voluntarily), it doesn't mean that EVERY single bank in the country is going to be comprehensively implementing it and rigorously checking all clients. Do you really think that every Swiss and Panamanian bank is just handing over everything they have on their most profitable and juicy clients willy-nilly? There are numerous 'mistakes' and 'clerical errors' along the way which mean certain information is never submitted to the regulator, let alone actually exchanged with other countries.
We all know how disorganised, incompetent, corrupt and useless many banks are and it isn't uncommon for them to not even request proof of one's tax identification number or a subsequent change of fiscal residence, as long as it fits the correct format that's the last you'll hear from them. Realistically, you need chunky deposits (low 7-figure plus which you cannot explain) concentrated in a single/or only a few institutions for the taxman to get a hard-on and really go for you.