I've heard that using a temporary spare will disable my ABS system. Is that true?

Yes. On many cars equipped with ABS and a temporary spare, an electrical connector inside the trunk or luggage compartment is attached to the valve stem on the spare tire. If you have a flat tire and remove the spare, you have to unplug the connector. This tells the ABS system that the temporary spare is being used, causing it to temporarily disable itself.

The reason why the ABS system shuts itself off when the temporary spare is in use is because the spare tire has a smaller diameter and narrower tread than a standard tire. This causes the tire to rotate somewhat faster than the other tires, and to brake differently. Were the ABS system not disabled, the difference in wheel speed and braking friction caused by the temporary spare would probably cause the ABS system to kick in unnecessarily when the brakes are applied. So disabling the ABS system prevents this from happening.

The important thing to remember here is to reconnect the ABS connector to the spare tire when it is returned to the trunk. If this is not done, the ABS system will remain disabled and unable to prevent skidding when braking on wet or slick surfaces.