Here are two articles about Delegation:
From these articles we know that delegation is related to Data source and functions. So I guess that as you use the correct Data source and functions, a List box may be able to show more than 500 items. And also, to make it easier to know what is and is not being delegated, the authoring experience provides blue dot suggestions when a formula contains something that cannot be delegated.
Yes, I agree with @v-yamao-msft.
Though in Tutorial it may say non-delegation but from time to time, improvement to the delegaeted behavior formula
and BLUE DOT is the best guide.
I encountered for the very first time and reported and since then, I know BLUE DOT will guide us of the latest improvement from PowerApps Group.
Have a nice day.
Currently listbox only supports 500 items. Please fix this bug and I do not see any other control with multiselect(I have seen one workaround to use gallery as multiselect but thats not a great way of using powerapp plus how to unselect). There are many scenarios where apps/businesses need a listbox that shows more than 500. I am currently stuck in one req. where products are 2k and I want to do multiselect filter based on multiple products. In asp.net/sharepoint I can easily achieve this. and this is a hard requiement for app to move forward.