Bulkify the Logic

public void afterUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) {
    Map<Id, Decimal> amountAdjustmentByAccountId = new Map<Id, Decimal>();

    for (Opportunity opp : newOpportunities) {
        Opportunity oldOpp = oldMap.get(opp.Id);
        Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won';
        Boolean justUnwon = opp.StageName != 'Closed Won' && oldOpp.StageName == 'Closed Won';
        Decimal amount = opp.Amount == null ? 0 : opp.Amount;

        if (justWon) {
            addAdjustment(amountAdjustmentByAccountId, opp.AccountId, amount);
        } else if (justUnwon) {
            addAdjustment(amountAdjustmentByAccountId, opp.AccountId, -amount);
        }
    }
    // ... apply the adjustments (next section)
}

private void addAdjustment(Map<Id, Decimal> adjustments, Id accountId, Decimal amount) {
    if (! adjustments.containsKey(accountId)) {
        adjustments.put(accountId, 0);
    }
    adjustments.put(accountId, adjustments.get(accountId) + amount);
}

This is Module 16's "Maps of sObjects" pattern combined with Module 6's ternary operator (defaulting a null Amount to 0) — one entry per affected Account, even if several Opportunities for the same Account changed in this one transaction.

private void applyAdjustments(Map<Id, Decimal> amountAdjustmentByAccountId) {
    if (amountAdjustmentByAccountId.isEmpty()) {
        return;
    }

    List<Account> accounts = [
        SELECT Id, Total_Won_Revenue__c
        FROM Account
        WHERE Id IN :amountAdjustmentByAccountId.keySet()
    ];

    for (Account acc : accounts) {
        Decimal current = acc.Total_Won_Revenue__c == null ? 0 : acc.Total_Won_Revenue__c;
        acc.Total_Won_Revenue__c = current + amountAdjustmentByAccountId.get(acc.Id);
    }

    update accounts;
}

This is Module 25's core pattern applied directly: one query (WHERE Id IN :...keySet(), Module 16's keySet()) and one update, regardless of whether 1 or 200 Opportunities changed in this transaction — exactly what makes this feature bulk-safe.