Is this with regards to parameter inputs? If so, you can use the choice with text parameter type to 1) allow the flexibility of users entering in new values and 2) selecting existing values from choices related to what they have typed in. See this demo app for example.

Yes, this is a good example!

I recognized that published parameters has improved a lot i 2021! 

one detail: the "existing data (choices)" changes every now and then in my case. 

Is it possible to populate "existing data (choices)" on the fly (i.e. python), or do I have to wait for something like a "startup published parameter"?

Thanks!