It’s not just an urban legend.
However, I don’t think that it related to the Vega.
The identical badge-engineed Buick Skyhawk, Chevy Monza, Pontiac Sunbird, and Olds Starfire of the '75-'80 model years were equipped with a 3.8 liter V-6 that was very large for the size of the engine compartment on these small cars. The result was that, in order to change the spark plugs on the right bank of the engine, you had to disconnect the motor mounts, attach a chain hoist, and lift the engine at least a few inches in order to be able to access those plugs.
After finding out how much extra a tune-up actually cost on these cars, a lot of owners opted to skip spark plug replacement–in an era when plugs needed to be changed every 12k-15k miles. These cars were poor-quality dogs to begin with (brakes typically had to be replaced ~every 12k miles!), but when you factor in engines that were running very badly after a few years due to lack of plug replacement, most of these cars went to the scrap heap very early in their lives.
Since the Vega had a 4-cylinder engine with decently accessible spark plugs, I don’t believe that the Vega suffered from this particular bad design element. God only knows that the Vega engine had other design problems, but poor access to spark plugs was not one of them.
Edited to add:
Another engineering/design error that comes to mind has to do with the Datsun SPL-310 and 311 of the late '60s. The engine, which was essentially a copy of the MG 4-cylinder engine, used dual side-draft carbs.
When my brother and I went to change the air filter at around 10k miles, we discovered that the placement of the air cleaner housing–only about 1 inch from the inner fender panels–made it impossible to remove the cover of the housing in order to replace the filter. So–in order to replace the air filter element on these cars, it was actually necessary to remove…a large number of bolts…and remove the carbs from the intake manifold.
Once the carbs were removed, then you could remove the air cleaner housing cover and replace the air cleaner element. Since this car was such a badly-assembled, unmitigated engineering/design disaster in many other ways, we opted to do the air cleaner element change only once. We sold this rusting hulk after ~ 3 years, and said good-riddance to a badly-designed rolling piece of junk.