I just stumbled upon an article on a German IT news website mentioning that you have to pay to be able to set (a still very limited number of) hyperlinks in your posts.

But I couldn't find any other sources mention it, except some dated articles referring field testing of such a restriction on Facebook.

Has anyone some info on this or even firsthand experience as a Facebook or Instagram user?

If you get to a certain number of people maybe. Or location or who knows what.

I have a YouTube page where I post scuba videos. I want to use it to fund a non-profit. I have two websites related to the video's plus a Facebook, and a bunch of other sites. When I make a post on my site it sends a teaser post with a link to those sites. I don't play anything. But I had to do a ton of jumping through hoops to do it. There are services that do this for a fee. Maybe the fee is for them to set something similar up. But from my understanding Facebook loves links because their crawler hits my site to take a snapshot of the post for their users, wink wink ai bots to crawl. It's the same thing for Google. The more they crawl the more they gain in training data.

It seems to be a new thing.
One Facebook user in the article forum told, that he just got the message a few days ago, that only 2 links per month are free now.
And the article editor answered to that, that this is likely a reaction to some countries establishing fees or other restrictions for links to news sites by now, e.g. Canada.
Meta already extended the rule to links to any sites in Canada, and might currently be further extending that to include all users, as I understood it.