Cost is an overloaded term, as is trivial.
When talking about execution plans, the estimated cost is computed by the query optimizer, as a way to choose one plan, or one small part of a plan, over another. The final plan has an associated cost computed by summing all the subtree estimated operator costs. A trivial plan is one obtained without going through cost-based optimization.
When talking about the plan cache, a separate cache cost is computed that is an input into how long the plan will survive in cache (normally as a response to memory pressure). The details of this computation, and what is considered a "zero cache cost" (or "trivial") plan has changed many times over the years. There are many exceptions, caveats, and restrictions, which again have changed quite often.
Trace flag 2861 was only effective for a very short time. It does not exist in modern builds. Sean Gallardy of Microsoft says in a blog post:
Here’s the low down, trace flag 2861 did exist for the briefest of moments in history. It was added in SQL Server 2000 SP3 (I can’t find the exact build number at this time) and subsequently removed before SQL Server 2005 shipped. That’s right, it existed for roughly two service packs of SQL Server 2000 before being removed, for good.
If you’re still using this trace flag today, it literally does nothing.
You can find some of the details (that were correct at the time they were written, to the best of my knowledge) in resources like:
