Hi littlewing
All modern kite bars have equal line lengths by default from the factory. The only exception that I know of was on an old F-One kite called the Volt or Revolt? ( around 2010?) who had back lines longer than the front lines by default.
Common sense says that your lines will stretch over time but this assumption is wrong. Lines are pre-stretched to the max in the factory, if you grab a brand new bar or a brand new set of lines and try to stretch them you simply won't be able to.
I recently held a kite maintenance/repair clinic and grabbed a brand new bar from the shop to compare the length difference with a bar that had only 2 months of use in my school.

The top bar with the sorter lines is the used one and the bottom bar with the longer lines is the brand new one.
Lines can also stretch but note that that they will always
shrink first. A common example of stretched line is if you are trying to relaunch your kite by pulling very hard on one of your back lines, this will stretch that back line that has previously shrunk over time
Front lines will shrink less as they are always under load. Therefore, front lines will eventually become longer than your back lines and your kite will backstall
This process happens very slowly so most people won't notice. It's a good idea to tune your lines every 20-30 sessions or so.
Kite manufacturers know your line length will change over time hence why they provide adjustments to lengthen the back lines and/or shorten the front lines.
Line shrinking is more of an issue in hot/sunny weather (Australia) as I noticed lines shrink less in colder climates.
If you grab any bar from any manufacturer when it's brand new and measure it's precise length, you will notice the lines have shrunk after only 30 days or so of use.
As James said, over time, this can reach well over 20cm of difference
Christian