Can't find kmod-fs-exfat package, especially for mvebu device. Is it (package) deprecated/renamed?
It is there, but hidden unless you select to also "build patented" packages.
The "build patented" default was changed in January to disabled, so you need to manually toggle that to enabled.
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=dc555d003c21679c8c94ac7f5c74cbd5cd089ae0;hp=03a00eeab38f6238b2c1f3b33583266797242eb5
exfat is patented by Microsoft, so that kmod-fs-exfat needs that explicit selection to build also patented stuff.
Global build settings in menuconfig
or directly in .config: CONFIG_BUILD_PATENTED=y
related discussion in Media Server On Lede Project - #10 by hnyman
Thank you for update, hnyman!
Does this actually mean that there will be no "patented" packages in official openwrt repo?
Yes, to my knowledge (for master and new stable releases), but the old 17.01 has the old defaults so patented stuff gets built for 17.01 release download repo, I think.
@yousong made the change, so perhaps he has better knowledge about the effects.