There are common properties of phycology shared by people. UI design and ergonomics rely on such properties. In psrticular, how people read text.

But I am speaking of myself only. From experience receiving well designed message comparing to the experience with badly designed messages.

I am a data point of evidence supporing my view. The opinion that "people don't read" is a complete speculation, without convincing evidence.

The real problem that many services simply not include the warning in the message.

OP’s claim was not that “people don’t read.”

It was that “[t]hey only read what they need to finish what they are currently trying to do.”

Those are two different claims.

Ok. When they need the code they will have to scan through a message like

    Do not share the code 3456 

and will read the words, because they read left to right.

The code should be in the same font as the rest of the text.

I can assure you that by now, my brain is conditioned to lock into the four-digit code as soon as it can, entirely ignoring everything around it, including the words to the left.

I’m an avid reader. But there are limits to what I can process, and our world has become so full of noise that it has become a coping strategy for brains to selectively ignore stuff if they feel it’s not important at the moment. That effect becomes even more pronounced as the brain deteriorates with age.

I do not believe that receiving such a message you will not notice the phrase.

And more so if you receive them constantly.

But of course, you are entitled to your opinion, even if it's wrong.