(Emphasis added.) Thanks for responding! I understand that you feel many posters fail to treat different versions of D&D as entirely separate games. As that is an important part of your philosophy towards multi-edition games, I can understand why that would be frustrating.
From the bolded section, it sounds like you are concluding that because you think the rules are clear, posters who disagree with you about what the rules say are likely engaging in motivated reasoning. That's not a definition of motivated reasoning that I can support--it privileges your opinion of what the rules mean over those who disagree with you. Indeed, it sounds like you're saying that because lots of people in the past have engaged in motivated reasoning, you're willing to assume that anyone who disagrees with your reading of a clear rule is likely engaging in motivated reasoning.
Obviously I disagree with your reading of the rule. If the mere fact if my disagreement is truly enough for you to dismiss my opinion as due to motivated reasoning, there's not much I can say to try to convince you otherwise. If it helps, I like that 5e calls for fewer climb checks than other editions, so I really don't think I have any ulterior motive when I nevertheless read the specific climbing rule as leaving identification of climbing complications up to the DM.