The two sentences that contain examples of contested usage are:
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"The postman needs to know who this letter is addressed to so it can be delivered." - This sentence contains a contested usage because "who" is traditionally considered incorrect in this context; "to whom" is seen as the more grammatically correct form.
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"And William also said that he did not understand why he couldn't play on the football team anymore." - This sentence uses the phrase "couldn't play," which can also be contested depending on the interpretation of past ability versus permission. However, it's less clear than the first example.
The first example is definitely a clear instance of contested usage due to the grammatical debate surrounding "who" vs. "whom." The second example might not fit as cleanly, so you could also consider only the first sentence as containing an uncontested usage.
If there are specific usages you need to evaluate further, please provide additional context!