I never said that the update causes issues with 3.18 kernels exclusively, although I might've subtly implied that. In fact, I suspect that similar issues can occur on other kernels with the updated sdfat driver, including kernels based on 4.4.
Updating kernels to newer major releases of the Linux kernel for android devices is very much easier said than done. That would require all of the drivers and other bits to be forward ported, which is extremely difficult, time consuming, and requires quite a lot of knowledge, and there is a very high chance of there being issues after getting everything to compile, and even more changes would be needed to fix such issues. Android device manufacturers and SoC vendors very often make very extensive changes to their kernels, and this is true for Samsung as well as others, making things all the more complicated. Almost (think >99.9%) everyone has to go with their vendors older downstream kernels as a result - if it was so easy to update kernels to newer major updates, no one or less people on fewer devices would still be using older downstream kernels. Many are still using even older 3.10, 3.4 and 3.0(!) kernels on older devices with backports. Drivers could also be written from scratch, but that has its own significant set of difficulties.
The last minor update for 3.18 was actually 3.18.140, released on May 2019 (
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/log/?h=v3.18.140), and it was updated for that long for the many devices out there running on 3.18. The 3.18 Android common kernel indeed continues to get updates, including some fixes to security vulnerabilities. On my kernel for the Tab A, as a compromise, I'm also applying changes from 4.4 as well as changes from the 3.18 Android common kernel and previously also, when it was still maintained, Linux 3.16.