Yeah - basically the ketchup of the day. Worcestershire sauce is apparently the closest extant thing, as it’s made in pretty much the same way - fermented anchovies.

I think the closest thing is probably fish sauce.

https://en.wikipedia.org/wiki/Garum#Legacy

Example: Tiparos (on the left) https://importfood.com/products/thai-sauces-condiments/item/...

Happens I can recommend tiparos as it's not brutally salty like others I've met. My guess is that this modern kind of fish sauce is useful (and I've heard it highly spoken of by a semi-professional chef) because it's high in glutamates which taste meaty (umami) but that's just a guess.

Curious, why Worcestershire sauce and not any of the fishsouces you can buy in a chinese/Thai/Vietnamese grocery?

In fact the Vietnamese fish sauces (which are made without adding any sugar or soy, and only made with anchovies and salt) are exactly like the garum and liquamen of ancient Rome.

GP probably meant "closest" in terms of lineage, not just composition. There's more of a Western connection to Worcestershire.

You can buy garum today, from manufacturers that claim to follow recipes hundreds of years old. The process is claimed to be unknown, but this is in fact not true: A couple of descriptions exist of the process, for example how it was made in Caesarea, in Judea, which also manufactured murex purple dye.

For me it is a mystery how a product made all around the Mediterranean in antiquity died out almost completely. Why didn't people in southern France, for example, keep eating garum until the modern day? Was it just changing tastes, sudden loss of necessary ingredients due to breakdown in trade, or recognition that parasites inside fish could survive the garum-making process?

It's a great question! One thing I might suggest is human migration displacing garum, or displacing the people that had a taste for it. Between "Antiquity" and today, you're as likely to find an immigrant population as a native one. I've been reading a lot about Roman history, and trying to answer questions like this, and many signs point to migration. The various -goths and Vandals in particular carved the Iberian peninsula (and southern France) out of the increasingly fragmented (Western) Roman Empire. According to the original sources, most of the garum in the Empire was produced in both provinces of Hispania. Considering the, uhh... "dynamism" of that place in the ensuing centuries, it sounds believable to me that garum simply fell out of favor culturally. Trade collapsed after the fall of the Western Roman Empire. So garum shipments from the Empire's largest garum-producing region simply stopped.

It is true however that there are still Italian and Greek garum producers, as I mentioned, who claim to have been producing since Antiquity. Though this is 100% unverifiable, there's little reason to think it's not true. The thing about garum production is that IT IS SMELLY! Very, very smelly. Roman law actually forbade garum production within a certain distance to the city walls. And as the Western Empire began to evaporate, the production of most specialty / delicacy items (think wine, garum, honey, cheese, etc) moved to "wealthy manor homes" which -- because of the danger of the roads -- slowly became the armed fortress fiefdoms of feudal Europe. So probably it just fell out of favor as time wore on, except in a small few exceptional places.

What does it taste like?

Umame, in a nutshell. Hard to really quantify, but I recommend trying it!