There are a few slightly different issues here.
Providing official sources
It is not required to quote or reference official denominational sources in answers, but it is strongly encouraged, especially if your answer is saying something that is not fairly well known. If you don't provide sources, you must be prepared to add them if challenged.
Previous discussion: Citing Sources in 2020/2022
I think this is more of the real issue: the personal beliefs of the members of this site are not on-topic. This site is about the documented teachings of established groups or historically significant individuals, not what any of us might think and reason. Any answer which is not clearly representing the views of an established group is liable to be deleted.
Most questions come with an explicit denominational scope, so it is then implied that all answers will represent that perspective. It's not necessary to always explicitly state the denomination of the answer. But sometimes answers will write something which seems out of step with the denomination. It's common for someone to write a comment on those answers along the lines of "What you've written here doesn't seem like what denomination X teaches, can you please add sources showing they really do?" About 10% of the time the author does, and we see that it is a genuine teaching, albeit a surprising one. 10% of the time they add sources and we can see that they've misunderstood the denomination's teachings. And the rest of the time they either comment saying they're not trying to represent that denomination or they don't respond, both of which result in the answer being deleted.
For the question linked about, it asks what non-Trinitarians think about a topic. This means that it's effectively a kind of overview question, and answers should be representing the views of the non-Trinitarian branches of Christianity:
- Jehovah's Witnesses
- Modalists and Oneness Pentecostals
- Arminianism and other historical non-Trinitarian groups
All answers to a question like the one linked should clearly identify which of these groups it is representing. If someone is presenting their own beliefs (which is after all the most common thing for answerers to do), then they still need to identify which perspective they are answering for, and be willing to provide official sources if requested. Answers which do not will be deleted, especially if it's not clear that the answer even is presenting a non-Trinitarian viewpoint. This is the case with the deleted answer - many Trinitarians would actually agree with pretty much everything it says!
Occasionally people come to this site who believe that basically everyone else's theology is wrong. While they are of course welcome to ask and answer on-topic questions, there will not be a place for their personal beliefs to be presented on this site. (Except for chat where any respectful discussion is allowed.) For any such person though, I would encourage them to humbly do more research: after all this time almost all theological options have been raised before. The attitude that everyone else is wrong is not a sign of someone who is well read, but instead someone who is quick to dismiss other people, who doesn't take the time to understand what other people say and teach.
The ongoing site cleanup
Lastly, yes, we mods do actually clean up a lot of old answers which don't meet the site standards. The community (not just the mods!) have over the years tightened expectations a little. Poorly referenced answers with questionable content are more likely to be deleted now than in the past. But they're never gone forever, they can always be undeleted if edited, and we're more likely to add a post notice to insightful but unreferenced answers rather than immediately deleting them.