First of all, you may have experienced some toxic management, which (sadly) is not very unusual. If that's the case, you may want to be more careful picking on companies culture on your next job search.
However, before we drop the hammer and call people toxic, I want to go over several checks.
First of all, you need to understand that managers are hired and getting their paystub from their employers just like you. In short and blunt way, they are working for the company, not you. So before being judgemental about management
exclude others from career opportunities, we need to ask ourselves the question, are they doing what they did for the sake of ruin your career, or just to better serve the interest of company.
Borrowing some of the examples you listed:
contacts and relationships (make excuses for not introducing others to key customers or partners, even if it would benefit her/him)
In certain cases, partnership can be very fragile and non-transparent. Imagine the case if Jack is not authorized to communicate with client/customer but he do:
Client: Hey Jack, this new product seems great, how much does it take you guys each?
Jack: hmmm, something around $15
Client: But your boss quote me for $50!
Jack can easily disclose sensitive info without proper knowledge about what to disclose and what not. In such case, it is in company's interest to avoid any contact between Jack and the client.
presence at tradeshows (be the one giving the talk, talk about others' work on their behalf if possible)
Company CEO's/managements present about their product on conferences or exhibitions regularly, which doesn't mean they take all the credit of the people who did the exact jobs. There need to be a person to present things to public, and it requires skills and overall knowledge to the product/company.
high profile projects (actively exclude colleagues from big projects, even if they would benefit from learning new skills or applying their relevant expertise)
Management get to decide the right personnel to join certain projects. And I must repeat, they are working for the benefit of the company, not certain employees.
Say Jane is for sure going to benefit from learning something on tech B which she's not very familiar with, but here's a project involve tech B which is mission critical and having a very tight deadline, then as management I would not have Jane involved in this project. There will be some bitter feeling for sure, but it is the management's job to make the call.
access to valuable assets (e.g. become a gatekeeper for some data or equipment which would be useful for other people as well, especially some who might make good use of it)
This might be the most common case that everyone experienced. And I shall repeat, maybe certain data access or equipment will be very useful or helpful for you, but managements are not working for you, they are working for the company.
Mike: Hey boss, I have the new payment verification flow ready, do you think I can get like 1000 rows of real customer data and use them for test?
Boss: No sorry, you have to ask the QA team to generate some mock data for you.
In this case, if management provide access, it can potentially result in a breach of PII, which can get the company into serious trouble. It is the management's job to prevent any potential problem like this.
And again, since I do not know your exact situation or situation you experienced, I can not draw a line on what's the intention of your management. However I would always suggest to think twice before judging the management's purpose was to
exclude others from career opportunities. After all, the ones who really bringing the toxic mentality can't really get that far on the management career route.