ほとんどの企業（または業界）ではそれが許されていないって英語でなんて言うの？

This is a very simple way to tell them that the practice isn't allowed in most companies. I hope that this helps! :)

For the most' part is another way of saying 'usually'. 'Permitted' simply means 'allowed'. These words come from the nouns 'permission' and 'allowance'.

The two sentences you see provided above are excellent ways to express to your listener that something in particular is not allowed in most companies. In the first sentence you will see the word permit and in the second sentence you will see the word let. Both of these words mean to allow. Both of these words are commonly used in our everyday conversation.

'Most companies do not assent to this.'' The word assent means to say yes to something.
Therefore, this sentence means that many companies do not accept or ''say yes'' to (this thing).

Another word you can use instead of ''not allow'' is disallow.
Therefore we can say, ''Most companies disallow this.''

Not accepted means its not allowed.

When talking about something that is usually not allowed in many companies/industries, you would like to know how to explain that fact. Above are a couple of examples which may be appropriate.

Something that is not allowed or accepted in a company or industry could also be called a 'company policy'. If this policy or rule is broken it could result in you being told off or reprimanded. The worst-case scenarios would be suspension or being sacked.

Often we can confuse our business world with our personal life. This is especially easy given our huge use and dependence on social media websites. Many people feel free to expose their personal life on Facebook, twitter or instagram. But, after meeting new coworkers, it's usually not appropriate to send all our personal photos, videos or show our political beliefs while pushing them onto our coworkers. We could say: "In the business world, we don't usually do that." Also, asking coworkers about very personal information is usually wrong, and we say: "That's not normally done in business."

There are lots of synonyms we can use to express that something is not allowed. 'Forbidden', 'prohibited' and 'not permitted' are acceptable ways to say this.

We can say that something is 'frowned upon' if it is allowed, but is looked at with disapproval and is best not to do.

To express that something is not allowed in most companies use the sentences above.
Most companies have general rules that are similar to industries.
The word 'most' explains that it is a majority of companies that hold this rule.

"Most companies are not okay with this pratice."

There is nothing wrong with your own example and I think you can easily use this phrase. Although, I would personally use the sentence I used first because it's a more fancy way to say it. The second sentence is just a simple version of your own example.