AFAIK this would require a plug-in or modification to the core codebase to implement. As a work-around, each topic has the green “progress bar” in the lower-right. You can always click the down arrow to be taken to the last post in the topic.
Also, in case you didn’t catch it from @pyro240’s response:
In other words, Discourse keeps track of your reading position. For example, if a topic has 40 posts and you read 30 of them, it remembers that. If the topic gets 40 more posts while you’re away, it will start you at #30 so you can pick up where you left off. I know that’s not a direct response to your query, but it’s useful and cool.