How often are authors not allowed to send their paper by email upon request?
I will speculate (and I'm guessing that a speculative answer is the best you'll be able to get to such a question) that the answer is effectively never. Specifically, I have never heard of a journal attempting to impose such a draconian restriction on authors, and moreover, I can think of two quite strong reasons why they would not do so:


Any journal that attempts to impose such a restriction is very likely to be blacklisted and boycotted by authors.
The restriction is unenforceable, since email communication is private and therefore a violation could never be detected by the publisher. In the world of dealmaking, savvy dealmakers know* that it is completely counterproductive to insert unenforceable clauses into a contract, since they will surely be ignored and their only effect will be to antagonize your counterparty and drive them to deal with a competitor instead.


* Okay, I don't actually know for a fact that "savvy dealmakers know ...", but since I know it and I'm not a very savvy dealmaker myself, I expect that it is even more obvious to savvy dealmakers than it is to me.

To summarize, if there is a journal forbidding authors from sending their paper to colleagues over email, it's a reasonably safe bet that the journal is a nonentity and can be safely ignored.