Is there a right and wrong way to use a forum
No - but the software is designed around a set of assumptions as to how it will be used.
If users expect the software to behave differently to this, then it can lead to frustration or confusion.
I think one thing you're missing is that sometimes people only use the forum to watch very specific threads. The Beardsley Zoo news thread(s) is a good example because it's been around as one thread for years, and certain members (in this case in particular BeardsleyZooFan, the main contributor of Beardsley Zoo content) really only log onto the site in order to read about this one zoo. When a thread is split off into chunks without any sort of announcement, these members will not know what's happened and thus will not be able to contribute to the new thread(s).
Again - my point remains that this is not how the forum software is intended to be used - it is expected that people will want to know about other content and will thus use the New Posts searches to discover it.
That being said - your points are not invalid, and there are plenty of other similar use cases I can think of where this all becomes an issue.
This is especially true of very large forums where there is a lot of content being created - even more so when much of it is on topics only of interest to specific subsets of users. That's basically the issue we have here - even so, it's a bit unusual to have people only watch a single thread rather than a whole forum topic, which would have avoided this issue.
Watching a forum topic is kind of the point here - if this is a topic you are interested in, you want to be alerted to new content created in that topic.
However, we also break our topics (country-based for zoo discussions) into zoos using thread prefixes - and so if you are only interested in specific zoos from a country and not all zoos - then there is currently no way to watch a prefix (another suggestion I've made to the developers!) and we we still don't have a workable solution.
I would like to clarify that I am not suggesting that there be some sort of announcement every time a new yearly thread is created, I am suggesting that when a thread that was previously not a singular year thread is suddenly split up and turned into year threads, there should be a post at the end of the latest thread informing people that the thread has been split and all the posts for the new year can be followed on a new one. Something akin to what was done when the Big Year thread from last year was split into four quarters, simply a post at the end saying it was split and posts have been moved and that the conversation will continue elsewhere.
Yes, this is a good suggestion - there is a clear differentiation between threads that are already split by year and so have an expectation that at the end of the year a new thread will be created - vs one that has been newly split into year-based-threads.
I have experimented with that today when I split the 10+ year old
Rotterdam Zoo - Blijdorp News thread - I posted in the original thread (which is what everyone was watching) about the newly created news thread.
I generally prefer not to do this because now we have a post from 2019 in a thread where the last post is (currently) 2011 - however, right now we do not have a better way to handle this issue.
I've made a suggestion to the software developers about copying the list of watching users to a new thread when moving posts - but I will also look at building some kind of generic notification tool which can be used to alert watching users about major thread changes like this.