I don't think that links are good answers since they can decay. Whether or not they are behind paywalls is irrelevant. But I do think that it is perfectly fine (indeed, encouraged) to give answers that are complete citations of relevant papers. For instance, my answer here. These answers are perhaps not useful to some amateurs who do not have access to academic libraries, but given that our target audience is professional mathematicians and graduate students, I don't think that is a problem. If someone wants to be involved with research mathematics, they need to find a way to access the literature (a large amount of which is not available on the internet, even behind a paywall).

I don't think that links are good answers since they can decay. Whether or not they are behind paywalls is irrelevant. But I do think that it is perfectly fine (indeed, encouraged) to give answers that are complete citations of relevant papers. For instance, my answer here. These answers are perhaps not useful to some amateurs who do not have access to academic libraries, but given that our target audience is professional mathematicians and graduate students, I don't think that is a problem. If someone wants to be involved with research mathematics, they need to find a way to access the literature (a large amount of which is not available on the internet, even behind a paywall).

EDIT: I feel a little embarrassed that I immediately got a bunch of upvotes for the answer I linked to! That wasn't my goal at all; instead, I just flipped through my answers and linked to the first one of this form I came across...