The Bug
Worlds with long names won't fit the toast that appears when you back up a world. MC-116661 had a similar problem where long names of advancements were cut off, but was solved by letting "Advancement Made!" and the name of the advancement appear separately, which could solve this problem.
Steps to Reproduce
Create a new world named "Super Long World Name".
Exit the world, return to the singleplayer menu, select it, and click the "Edit" button.
Now click the "Make Backup" button and wait for the world to be backed up.
Look at the backup toast displayed on the upper right side of your screen once backing up has finished.
Take note as to whether or not world names are cut off in backup toasts if they contain too many characters.
Observed Behavior
World names are cut off in backup toasts if they contain too many characters.
Expected Behavior
World names would not be cut off in backup toasts if they contain too many characters.
Attachments
Comments 9

Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.
Can confirm in 1.17.1.
I am able to confirm this behavior in 1.18 Pre-release 1 and below, have provided additional details regarding this issue.
The Bug:
World names are cut off in backup toasts if they contain too many characters.
Steps to Reproduce:
Create a new world named "Super Long World Name".
Exit the world, return to the singleplayer menu, select it, and click the "Edit" button.
Now click the "Make Backup" button and wait for the world to be backed up.
Look at the backup toast displayed on the upper right side of your screen once backing up has finished.
Take note as to whether or not world names are cut off in backup toasts if they contain too many characters.
Observed Behavior:
World names are cut off in backup toasts if they contain too many characters.
Expected Behavior:
World names would not be cut off in backup toasts if they contain too many characters.
This issue was present in 22w05a, but no longer occurs in versions above or equal to 22w06a. With that being said, this issue has been fixed in 22w06a. To reinforce my claims, I've attached two screenshots demonstrating the difference in behavior between these versions.
[media][media]I cannot reproduce this behavior in 22w18a. See my above comment.

Can confirm fixed.
Confirmed for 1.13.1.