I think as always I have chosen the wrong one...
I wanted to add another test parameter where the remainder was
Kind of "easy":

param1 = {"estimators": [("cBOSS", cboss, 5), ("CIF", cif, [3, 
param2 = {**param1, "remainder": "passthrough"}

getting the error:
AssertionError: get_params result of ColumnEnsembleClassifier (x) does not match what was passed to set_params (y)

is there any example where I can fix this correctly?

I applied your changes but I think I need to do something with set_param because I still get the error:

get_params result of ColumnEnsembleClassifier (x) does not match what was passed to set_params (y). Reason for discrepancy: [remainder].len, x.len = 4 != y.len = 11

I need to somehow add a set_params when I pass the "remainder": "passthrough" but I am not able to localize it.

The reason maybe that remainder is not correctly set or gets overwritten?

This might be an unreported bug.

The general requirement is that no parameter that was set by __init__ and is of same name as the init args should ever change over the object lifetime.

The reason maybe that remainder is not correctly set or gets overwritten?

This might be an unreported bug.

The general requirement is that no parameter that was set by __init__ and is of same name as the init args should ever change over the object lifetime.

If you can give me a bit more details on how to fix it, I'll try.