I want to make a ginormous-AC character, but I am not sure of the rules about shields. I want to have two shields held at once, with no other weapons. I know this is against the Player's Handbook: "only gain the bonus from one shield at a time". But dual-wielding shields (with no other weapons) works for the fire giant dreadnought in Volo's Guide to Monsters, so has there been a rules update, or is this a monster-only thing?
The ability is monster-only
The Fire Giant Dreadnought's ability is an instance of a "specific beats general" rule. Generally, a character can only gain the benefits of a single shield. The Fire Giant Dreadnought has a specific rule stating that it can benefit from dual shields, so it can do so but this doesn't apply to any monster or character without such specific rule.
No change to the general shield rules are in the Player's Handbook Errata.
House ruling the shields is always an option, but may cause poor balance especially on lower levels when sources of AC are very limited. DnD 5e is designed with the principle of bounded accuracy (see for example this answer, meaning that AC and attack bonuses scale slowly with growing level. An extra shield will put one's AC several levels ahead in relative defense, and significantly reduce the threat level of enemies who now need to roll very high (possibly even natural 20) to score a hit in the first place.