When creating a resource pack, previously you were able to change the 'end.txt' file so when you defeat the ender dragon and step through the end portal, you would see custom text in place of the end poem. This stopped working as of 1.13 and instead just shows the default 'end.txt' regardless.

Usually the 'end.txt' file would be placed in the '<resource pack folder>/credits' folder.
This bug also applies to the 'credits.json' and the 'quote.txt' files it seems.