This is a compatibility fix which allows Alert Count to work on forums running Defer JS, TaigaChat* (keep reading), and certain other add-ons and styles which the previous version was incompatible with.
Although it is a huge code overhaul, it does not add any new features. So if you are not having compatibility problems, you do not need to bother upgrading, although it shouldn't hurt anything if you do.
Compatibility with add-ons like TaigaChat is not enabled out of the box! You MUST go to [Options -> User Alerts] and enable "aggressive mode" for compatibility with TaigaChat or any other add-on which modifies the page title. Aggressive mode allows Alert Count to aggressively keep exclusive control of the page title so that no other add-ons are allowed to change it.
Aggressive mode is turned off by default because it results in marginally more client-side script execution in order to combat the other add-on, and that extra work isn't necessary if Alert Count is the only add-on that manipulates the page title. So only use aggressive mode if you experience problems with the title updating correctly on some or all pages of your forum.
Let me know if there are any issues in this version.