what are the things i will miss from plex’s pre-enshittification?

it will still shows stuff in the library even if it failed to pick up the metadata.

for jellyfin, folder structure is kinda important for auto detection to work.

For shows, you can organises your files like this:

series-name-a/
    season-01/
        episode-01
        episode-02

You can check out the doc, it is more detailed