This link:
https://blog.twitter.com/developer/en_us/topics/tools/2018/discontinuing-support-for-twitter-kit-sdk.html
talks about why they are discontinuing support for one of their SDK tools.  It will still continue to work, it's just hat twitter themselves won't be contributing to it anymore.
On the link:
https://developer.twitter.com/en/pricing.html
you can get details on exactly what they're charging for, and how much, though it's not laid out in a straightforward method, you need to select the various features, and their level to see what the fees are.
It actually looks like twitter has been targeting fee based services for quite some time now.  As far back as 2010, twitter was charging for access to historical data for researchers, commercial developers who wanted back history, and the like, so this isn't anything new. Last march, twitter had run a series of surveys, asking if users would be willing to pay for different components of the twitter landscape.  I didn't find anything breaking down what those responses were, but it isn't surprising that the questions were being asked.  After all, every company has a need to make money, or they won't remain a company for very long.
But, as far as the impending charging for access goes:
https://www.xda-developers.com/twitter-new-api-third-party-clients/
seems to indicate that the fees for a standard twitter client aren't trivial, and seems to contradict what I read earlier about clients simply not being able to get realtime updates.  It appears more digging is necessary.
Apparently, twitter is trying to herd users into using official twitter clients only, (or the web site), but as of now, I can't say whether this is true or not, since there seems to be more knee jerk reactions to the announcement than actual facts.  I suspect truthful information will slowly filter it's way into the general populous over the next few months, then we'll know for sure what the deal is, but for now, just keep using the client of your choice, and wait until it breaks ... (if it breaks) ... before spreading possibly untrue information about the change.