Why warn about evidence?

Right, that's what people call evidence.

If memory serves, naming it ev avoids warnings in Scala 2, though I don't know whether Scala 3 honors that convention.

Alternatively, simply apply @nowarn.

Calling it ev still warns:

[warn] -- [E198] Unused Symbol Warning: /Users/tavin/work/exercises-853_962/5/PolyFamily/src/test/scala/polycheck/helpers.scala:25:34 
[warn] 25 |def hasStandardForm[P](entry: P)(using ev: PolyFamily[P]): Boolean = entry.asMatchable match
[warn]    |                                  ^^
[warn]    |                                  unused implicit parameter

Certain familiar types are exempt from this warning, as are memberless types (“marker interfaces”).

Comparable, for instance, triggers it. If nothing gets compared, perhaps the constraint has become unnecessary, or perhaps you neglected to make a comparison.

scala> def inspect[A: Comparable] = "57".toInt
1 warning found
-- [E198] Unused Symbol Warning: -----------------------------------------------
1 |def inspect[A: Comparable] = "57".toInt
  |                   ^
  |                   unused implicit parameter
def inspect[A](using evidence$1: Comparable[A]): Int

I believe you have to mark it unused in this expanded version:

scala> def inspect[A](item: A)(using @unused witness: Comparable[A]) = item.toString
def inspect[A](item: A)(using witness: Comparable[A]): String

That's somewhat cleaner than @nowarn, although the latter accommodates context-bound notation.

(I suspect name-based checks date to before selecting

-Wunused:explicits,implicits,synthetics

In fact, synthetics isn't available in Scala 3 yet, only Scala 2:

  -Wunused:synthetics               Warn if a synthetic implicit parameter (context bound) is unused.
  -Wunused:nowarn                   Warn if a @nowarn annotation does not suppress any warnings.
  -Wunused:params                   Enable -Wunused:explicits,implicits,synthetics.

That arrived near the pandemic's beginning, presumably as a way of filling the hours.)