Unfortunately Ian is right. Even if the relay owner does not normally store the IP address, the court can force it to secretly start logging the IP address and you won’t know until it is too late. This is what happened when Swiss courts forced Proton Mail to start logging the IP address of a climate activist which lead to the activist’s arrest.
Unfortunately, even when people get arrested or worse when their IP is exposed in cases like these, Delta Chat does not acknowledge this risk or take the risk seriously, as you can see from the FAQ which downplays the risk of exposing your IP address and wrongly suggests that it can’t be used to identify you:
IP Addresses are needed for connectivity and efficiency. They are neither persisted nor exposed. Note that the IP Address is not like a detailed address you give to a delivery service, but much more coarse, often defining region or country only.
As this is just how the internet and other messengers work by default, we do not offer options here or ask upfront questions.
Ian is also right about traffic correlation analysis attacks.
You can read more about some real world traffic analysis threats here, which pose a threat to many messaging apps:
About your other question
If Multi-Device is enabled then messages will not be deleted from the server immediately when you download them so there can be a few weeks of message metadata on the server like message size and timestamps when the relay is served the court order or the police seize the server. But the police can’t read your messages unless they also get your key for example by stealing your phone.