I want to know why DSN rate limits apply to errors but not to transactions (or spans).
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DSN rate limits apply to error events only, not transactions or spans. That’s expected.

Set them under [Project] → Settings → Client Keys (DSN) by selecting Configure a key. See Rate Limiting.

To control transaction/span volume, lower the SDK tracing sample rates (tracesSampleRate / tracesSampler), drop events in the SDK with beforeSendTransaction, or use project inbound filters.

For more detail, see Manage Your Span Quota.