Hello @Paolo Scomparin
I just got confirmation from pm of both standard Direct Line channel and direct speech line channel.
For the standard Direct Line channel(the document you mentioned above), and individual message is stored up to 24 hours, and the record of the conversation up to 21 days after the last message in the conversation.
For Direct Line speech channel, we don't store any conversation at the server end, it's differnent from standard channel, please refer to below process:
Reference sample: https://learn.microsoft.com/en-us/azure/cognitive-services/speech-service/tutorial-voice-enable-your-bot-speech-sdk
For your second question, the document you mentioned is for Direct Line Standard channel not for Direct Line Speech.
Hope above helps! Please let me know if you have more questions and need further help.
-Please kindly accept the answer if you feel helpful, thanks.