Windows has a "sort by date" option I'm used to where it lists all items within folders instead of the folders themselves. For instance, if I open a folder only containing 10 folders with 10 files each inside and sort by date modified on Plasma, it will display the 10 folders. On Windows, it'd display the 100 items within every folder sorted by date. Is this possible on Plasma?

Are you talking about the "Folder View" desktop or about Dolphin? Also please see the red text in my signature...

In Dolphin, when in the Details view mode with Expandable folders enabled (Configure Dolphin > View Modes > Details), you can expand each folder and then sorting by the "Modified" column will sort them by the date they were modified - sorting the top-level folders by date, and then sorting the files within each expanded folder. (if you want to sort by other date columns such as Created or Accessed, you can add those by right-clicking on one of the column headers and checking the other columns you want to add)