Hello,
Thanks for your feedback.
check_date (90 000) is indeed more common than survey:date (30 000) in Poland despite having 2 meanings (before StreetComplete, it was only used for remote check like a website or a social media, for StreetComplete it's a survey).
check_date is also used to describe a check of one tag (but survey isn't a tag).
As you said, the previous mapper explicit describe it's a survey and here is the 5 only occurences.
it's why I have changed it into a tag documented and that don't lost the fact it's a survey. 
Regards,
Marc