So we can't eat them and people who liked the tallow-fried ones don't enjoy them as much (?)
Seems like as of a bit earlier this year, they still contained animal products, but this is in the US (in Canada, for example, it's vegan ingredients only).
They weren't “flavoring them with beef juice”, they were frying them in tallow. And AFAIK they didn't stop because of a lawsuit by vegetarians, but because of nutrition (saturated fat) concerns, and they now use other flavorings to attempt to simulate the flavor imparted by tallow:
https://www.wired.com/2014/07/whats-inside-mcdonalds-french-...
They actually did both.
https://www.thoughtco.com/mcdonalds-french-fries-still-not-v...
> But when they switched to vegetable oil, the fries were no longer as tasty. The solution was to add natural beef flavor to the spuds.
> Admitting that the fries were coated in beef flavoring, McDonald's settled for $10 million, with $6 million going to vegetarian organizations.
https://www.cbsnews.com/news/mcdonalds-settles-beef-over-fri...
> The vegetable oil used to prepare the fries and hash browns was not pure, but contained essence of beef for flavoring purposes. Many Hindus consider cows sacred and do not eat beef.
If you go to McDonalds and all your friends get a meal, you're not going to be all that satisfied with just fries and soda.