YC does an 'open source panel' every batch where people come to hear from founders of successful open-source startups in order to learn the ropes. I've attended 4 of those by now (I think), so I have a sense of what gets said about this stuff from within the YC space at least.
I haven't heard anyone talk about ways to "pay-wall necessary features" or otherwise exploit users into paying. On the contrary, there's a lot of talk about how critical it is to be transparent and fair with your community. The consensus is that things tend to go well if you do that and badly if you don't.
The focus for these open-source companies is finding a natural way to carve out the free vs. paid parts of the space. By 'natural' I mean something that is a good fit for the domain and that both sides feel is fair. The free users know it's in their interest for the company to make money because that's how the whole thing is sustainable. People just need to feel that the paid product is one that it makes sense to charge for and that it's a fair exchange.
The most common way to do this is to open-source the product and offer a paid cloud offering. There are other approaches which I remember I found rather interesting, but unfortunately I forget the details because thanks to HN I never remember anything anymore!
But the main thing is no, not only are these founders not looking for ways to screw their open-source users, the seasoned ones are advising the junior ones to shy away from the slightest trace of that. The model by which an open-source company is making money needs to be as transparent and unimpeachable as possible.
One downside is that it's hard to get this right from the beginning, and changes can be messy. From what I've heard, the consensus is that if you keep a good relationship with your community at every step, and preserve transparency as an invariant, then it's at least possible to explain why a change is necessary and get through a messy phase that way.