your previous contrib was for 2.7, you submitted against master, but described as 4.0
what's the right choice?

It was on 2.7 branch but fabpot merged it into 3.4/4.0 branch. Now it's the same, only for 3.4/4.0 branches

Isn't the real issue that we try to display false in the first place?

@xabbuh what do you mean?

<button type="{{ type|default('button') }}" {{ block('button_attributes') }}>{{ translation_domain is same as(false) ? label : label|trans({}, translation_domain) }}</button>

Here we always display the label variable. Why do we do that even when it is false.

Print false is same as print nothing because this I only check for label false one time.
If you don't print label when it's false (as you suggest) we have two options with the current code: check 2 times if label is false or execute previous ifs for nothing and finally ask if it's false. Maybe both are clear but less "microefficient".
We are writting more than the real impact of this change :)

Thank you @TeLiXj.