Does disabling a tax affect my old invoices?

No, disabling a tax does not affect old invoices and transactions in any way.

Invoices and transactions that used the tax before it was disabled remain exactly as they are — calculated values, journal entries, and financial reports do not change.

Disabling only affects new transactions — the tax disappears from all dropdown menus and cannot be selected when creating any new invoice or transaction.

If you want to use the tax again you can re-enable it at any time from Settings → Tax Settings.