Did the population at the time of the 1921 census think or understand the details given would be confidential for their lifetime.
I very much doubt it.
The majority of previous census had been released on average after 80 years some considerably earlier.
There was not and never had been any privacy law in the UK that is a figment of the imagination of people in the 21st century.
In any case what is confidential about the details of the 1921 census
1921 Census holds details of
Name & Surname
Relationship to head of household
Age, sex, married or single for children under 15 whether parents alive or dead
Birthplace and nationality
Education (i.e. whether attending school full or part-time) Occupation, Employer & place of work
Married men, widows & widowers also had to state the number and tick a box for the age of each living child under 16 years old
As can be seen from the above no personal information was asked, no questions about medical condition etc.
The Irish, Welsh and Scottish census differed slightly and also asked about language.
Practically all of the above information would have been common knowledge to the family's friends & neighbours, unlike today people talked to each other and shock horror often went into each others houses, they lived in communities.
Cheers
Guy