The new Shadow client v1.11.0 introduces the possibility to set the depth of message storage in terms of time – this in addition to the same in terms of number of messages in a chat (which option has long been available and remains to stay). All messages older than the specified amount of time are automatically deleted. This is local to your client and applies to all messages in all chats at once, as opposed to the “Disappearing messages” functionality applicable to individual chats. Of course you still can keep messages forever if you wish.
The setting is accessible via the Storage -> Keep messages menu.