KiCad V5.1.7 is now over 3 years old.
And the amount of changes / improvements in these 3 years is astounding. Back then KiCad was still quite rough in some areas and now it is maturing. An important improvement is that now all files are based on S-expressions, and all used schematic symbols are now saved inside the schematic itself. If you want to migrate KiCad V5 projects to V6 or later, make sure you have the [Project]-cache.lib file available. The Icon change in V6 was a big thing that needs getting used to, and there are hundredths of new features and small changes. the interactive router is for example constantly being tweaked and it’s algorithms work better in subtle ways. You can scroll though the next two threads for an overview of changes:
And in a few months (Expected February 2024) KiCad V8 is expected: