From the announcement mailing list:
I've just uploaded a version of OpenSSL to unstable that disables the TLS 1.0 and 1.1 protocol. This currently leaves TLS 1.2 as the only supported SSL/TLS protocol version. This will likely break certain things that for whatever reason still don't support TLS 1.2. I strongly suggest that if it's not supported that you add support for it, or get the other side to add support for it. OpenSSL made a release 5 years ago that supported TLS 1.2. The current support of the server side seems to be around 90%. I hope that by the time Buster releases the support for TLS 1.2 will be high enough that I don't need to enable them again.