I've implemented a new feature in Lineage OS Pro¹'s keyboard driver to be able to use Fn+Tab as Alt+Tab as this is most convenient for multitasking with two thumbs.
This thread is to track that issue specifically so please stay on topic.
I'm trying to contribute this back to official Lineage repository.
Here is my change request on Gerrit:
https://review.lineageos.org/c/LineageOS/android_kernel_fxtec_msm8998/+/315155
There was also a pull request on GitHub but that's just going to be closed as the GitHub repositories are just mirrors I guess:
https://github.com/Linea