https://github.com/arter97/exfat-linux/releases
This is already in the mainline kernel as of version 5.4.
I understand https://bugs.gentoo.org/712916#c1 that we can close it as fixed/obsolete. Please test the described solution and reopen, if I misunderstood you.
(In reply to Mike Gilbert from comment #1) > This is already in the mainline kernel as of version 5.4. I mount a 5.5.9 and it's not there if I haven't stoned.
You need to enable CONFIG_STAGING and CONFIG_EXFAT_FS in your kernel config.