RFC7457[1] and Wikipedia[2] offer an overview of many of the attacks on older versions of TLS. Some of those attacks have been mitigated to varying extents in implementations of the affected versions. TLSv1.3 is meant to resolve completely as many of these issues as possible.

When using older protocol versions, it can be complicated to validate that the TLS implementation you are using has the necessary mitigations in place. It can be complicated to correctly configure TLS to minimize the effects of known attacks. Doing that properly requires a fair amount of research, threat modelling, and risk assessment both for yourself and on behalf of anyone accessing your website or service.

IME, TLSv1.2 is still a big chunk of legitimate web traffic. It has been steadily dropping since standardization, and TLSv1.3 is the majority by a wide margin from what I can see. I wouldn't be surprised to see some websites and services still needing to support it for a couple years more, at least, depending on their target audience.

[1] https://tools.ietf.org/html/rfc7457

[2] https://en.wikipedia.org/wiki/Transport_Layer_Security#Attac...