First of all, this is a newly introduced configurable setting of
newuser_max_replies_per_topic which can be changed to every forum owners' taste but defaults to 3 for safety.
While we have always had global post limits, this new per-topic limit is in place because it is very easy for a new user to come in and ruin a single topic by simply posting in it over and over -- even with the "right" intentions. It's even easier with the "wrong" intentions. Just create a new account, be super argumentative, reply to every single person to replies to you at great noisy length telling them just how wrong they are, and.. destruction of topic.
This is particularly true in flat discussions, where all new posts live at the same place, the end of the topic.
One thing that bugged me with this situation is that new angry users "win" -- they successfully get moderators to close a topic just by signing up due to some topic that is a specific personal axe-grinding vendetta of theirs, then being excessively noisy and combatative in it. Primarily manifested as an endless stream of replies.. remember that Discourse is designed to be stable even if moderators aren't around to intervene.
More broadly, if you can't say your piece in 3 individual replies to a topic, respectfully speaking... you ain't trying very hard.
So as the new user types their 4th reply in the same topic, they will now see
You have reached the reply limit for this topic
We're sorry, but new users are temporarily limited to 3 replies in the same topic.
Instead of adding another reply, please consider editing your previous replies, or visiting other topics.
As for support topics, it is more correct to edit your previous posts to include the requested information rather than posting many new replies in a row, bumping the topic over and over and spreading information that should be condensed together into a massive vertical sprawl that everyone has to piece through.
In the specific case you cite, starting a new topic was actually the right thing to do, rather than continuing to post in an unrelated topic, which would force the mods (me) to split your posts out into a different, new topic. So I would say the new user post limit worked.
(Unfortunately there were 3 other nearly identical topics that already existed, which we try to highlight in a pop-up as you type the topic. The algorithm is crude though and only matches on title so it might not have been a good title match.)