Is a "typical" jam sweeter today than the typical jams of centuries ago?

On the contrary, most of the older recipes that I've seen for jams use more sugar (60g per 100g or higher is common) than is popular in some of the newer products, because sugar has a preservative effect. But most likely pre-refrigeration and pre-iceboxes you simply wouldn't have kept multiple jars open at the same time; you'd use one at a time as quickly as practical, and keep your reserve stock seals unbroken.

Industrially refined pectin has actually reduced the amount of sugar necessary to make a good preserve. A number of companies also now make "all fruit" spreads, but they're controlling the sugar ratio with knowledge from food science, by analyzing how much sugar is in the non-primary fruits like grape and apple juice.

60 grams of fruit per 100g or more are available but by traditional standards are quite a luxury, even though I would agree that such products often taste better; they owe their existence to better food science, however, and a little to refrigeration.

A lot of widely-distributed brands still use sugar ratios that are pretty high, including Welch's and Smucker's, but you may find higher fruit ratios in boutique brands.

Cane sugar or corn syrup is a relatively modern invention. Prior to these people did not have such sweetness available (other than fruit or honey), or used beets, carrots etc. for increased sweet flavours

Many cultures simply boiled fruit in an earthenware pot until it reduced to a sufficient thickness, and then put on the matching earthenware lid and seal it with wax, pitch, or animal fat. In a cool climates this lasts through the winter. This method was used with wild fruit during WWII in Europe due to no sugar being available

Roman and Greek records cite fruit jams made with fruit and fermented honey (melomel = marmalade). They also used pure honey, strong wine, and even vinegar!