Correct, there is less opportunity to overload/hook into/touch Invision Community 5. If you can give me some examples of why you'd want to overload those methods, we can help guide you to newer tools or understand why there is a need and consider adjustments to the dev toolkit.
Monkey patching (code hooks) were convenient but the cost was very high in that we couldn't significantly alter our code without destroying most existing modifications causing WSOD, ISE500 or other errors on client communities. As PHP 8 becomes more strict about typing, return types, etc - a simple function signature change could break modifications.
Clearly, allowing almost every single class and method to be overloaded is not something that we could continue doing.
We also want to be a little more protective of some of our UI and flows. We want to build a toolkit that lets you build amazing add-ons and extra functionality but it will mean there is less scope for smaller apps that change some of our existing functionality.
The good thing about these blogs is that we get to have a conversation and learn from each other.