The American frontier has a specific definition. It refers to the previously unsettled land West of the Mississippi. The frontier closed over 130 years ago - it’s not a fluid definition. See this article for details:
https://www.bu.edu/articles/2018/the-american-frontier-shapes-us-today-bu-researchers-say/.
By all definitions, “Frontierland” should be locked into the period from about the 1840s through the 1890s. Now, that being said, the notion of the “frontier” and its mythos is not necessarily time-locked. Like others have said, when polled, people in 2024 would probably provide responses like “wilderness” or “rugged mountains”. What they’d really be thinking of is the American West, which obviously still exists, as opposed to the frontier.
If Disney would like to add Cars, and given that they already added TBA and updated CBMJ, and just go for the “rugged wilderness” look, it should just consider changing the name of the land to “Westernland” or something along those lines. Thematic consistency should at least tried to be upheld, and if that means shifting the theme and name of Frontierland, so be it. I'd rather that than a mangled, incosistent husk of old Frontierland.