Around 1970 in Britain, was this use of 'shall', in 'You shall go (=I let you go)', already out-of-date in daily conversation?

A striking grammatical difference between BE and AE is the various uses of auxiliary verbs (now, modal verbs) of will and shall. When I was a high school boy studying English without any chance of speaking English, we were taught in the English grammar class a special use of shall, used particularly in the second and third persons ‘to represent the will of the speaker’ rather than that of the subject as in ‘You shall have the money tomorrow’ often paraphrased into ‘I will give you the money.’ Some more examples are found in Collins Grammar on the web:

• The president shall hold office for five years.
• Member states shall decide the conditions for granting access to the labour market for the applicant.

But I am not speaking about this use usually seen in regulations or legal documents, but sentences to imply a command, promise or threat made by the speaker. Some examples are cited in Wikipedia:

• You shall regret it before long. (speaker's threat)
• You shall not pass! (speaker's command)
• You shall go to the ball. (speaker's promise)

And another is also given in Leech’s Grammar (1987).

• Good dog, you shall have a bone when we get home.

These four examples suggest that this is in actual use even today. But, from my own long experience of studying English, I have assumed that around 1970 in Great Britain, this special use of shall was already out-of-date or old-fashioned in daily conversation or informal writings. Can I have some guidance about my assumption, or some information or data on this special use of shall?

If we take Google Books as an indicator, usage of the expression “you shall”, much in vogue before, has been visibly decreasing since the beginning of the 20th century, especially after the 1950s.

According to Lexico however, usage of “you shall” is not defined as “out-of-date”, but it is probably more formal, literary.

However, when it comes to expressing a strong determination to do something, the roles are reversed: will is used with the first person, and shall with the second and third. For example:

• I will not tolerate such behaviour.
• You shall go to the ball!

In practice, though, the two words are used more or less interchangeably, and this is now an acceptable part of standard British and American English; however, the word shall is now seldom used in any normal context in American English.

Google Ngrams search term shall: eng_gb_2019,shall: eng_us_2019,should: eng_gb_2019,should: eng_us_2019 Shows broadly that the fall in frequency of the use of "shall" in written English has dropped away to 20% of what it was prior to the Second World War. The drop in frequency in BE has been pretty regular.

Interestingly, there is a similarity that can be seen in the use of "should."

The use of shall and should remains consistent in legal language, where it comes close to the meaning of "must" (but allows for exceptional circumstances.)