we get the whole 20th century pace of innovation and we still have the internet, smartphones, mRNA vaccines, etc. but just no cars?
This is what I had in mind.
Hmmm... that's the hardest of the 3 scenarios
Scenario 1: Alcohol becomes a luxury good primarily for the 1% and ceases to be an important subject
Scenario 2: 1900-era social values get frozen in time, possibly for centuries, meaning Prohibition is almost surely still in effect in 2020 in this world
Scenario 3 (your scenario): In order for there to be the internet but no cars, you need a world where mechanical engineering hits a roadblock in the late 19th century and stagnates (like it did in the real world 1970's), while electrical engineering takes off much faster than it really did. This probably involves even more powerful railroad monopolies colluding with government regulators to stifle internal combustion engine research or if cars are invented but never widely used, to shut down highway construction. It would be overwhelmingly unlikely that commercial flight or the space race ever happen. This slows globalization, and petroleum would be relatively unimportant.
Investment is instead directed toward electrical innovations and medicine/biology, both of which are more critical needs with slower travel. In the extreme case, we have wireless electricity in the average American home in 1910, a Spanish Flu vaccine, and software engineering plays a role in WWI. Speaking of WWI, without cars there are no tanks and likely no aerial bombing, so there is probably just one looooong 30 year siege somewhere between Paris and Berlin rather than a separate WWI and WWII. The USA would likely want nothing to do with this and have little reason to feel threatened by it.
Coming back to the US domestically, farm families would be more isolated and the process of integrating rural areas into generic American culture would be much slower. Importantly for this question of alcohol consumption, the independence of early-mid 20th century young adults from their parents would be greatly reduced.
Put this all together and it strongly suggests a more socially conservative culture overall, particular when it comes to youth issues, so I doubt the shift from adulthood at 21 to adulthood at 18 ever happens to begin with. In most states, the drinking age is still 21 in 2020, but so is the voting age, because the age to act independently of your parents in any meaningful way is 21.
Prohibition probably passes sooner, but cities will grow faster in this world and they will eventually have the votes to repeal it. I could see it taking 39 years instead of 13 though. The 10 most rural states would probably still be dry in 2020. I could see a couple of the most urban states, particularly New York, allowing teenagers to legally drink in this world.