[LG V10 General] AnonVendetta:
I found 2 solutions which work on MM and Lollipop, and they don't involve replacing system files. One is an app, the other needs Xposed. I no longer recommend using Link2SD with MM, since it seems to be incompatible anyway. Some people did manage to get it working, but still no obb linking. Here are 2 superior solutions:

1. XInternalSD:

Download via Xposed Installer, activate module, reboot. Open a root file manager, go to /storage/, look for the folder that goes to your external SD's 1st partition. Open XInternalSD, where it says 'path to internal SD card', make sure it contains the right path to your external. The point is to trick certain apps (mostly games) into thinking your external is the internal storage. Check the box that says "full access to external SD". You can either "enable for all apps", or just certain apps. I recommend only for apps where obb linking doesn't work. Go to /storage/<external path here>, go into Android folder, create obb folder if it doesn't exist. Next, install your games. If you get them from Play Store, you will need to manually move the obbs to the folder above *before* running the app. Make sure you move the obb file itself *and* the folder above it. As an example, the game "Secret of Mana", my path looks like /storage/<external>/Android/obb/com.square_enix.secret/main.201410302.com.square_enix.secret.obb. When you have finished moving, open XInternalSD, tap "enable for apps", put check beside your app, then OK. Then you can run the app, this method should work for most regular apps and games. The downside of this approach is that you can only force an app to store most/all of its' external data on SD. External data is defined as anything that is stored on internal or external storage. All other parts of an app usually go into /data.

2. Apps2SD (search in Play Store):

There are free and paid versions (2nd is called Apps2SD Pro). I think free version has same features as Pro, but with ads. You can deal with them, block ads with the hosts file, or use Lucky Patcher to patch out the ads/create a modded APK with no ads. I recommend you buy pro version, it has so many features, and is way better than Link2SD, and is MM/Nougat-compatible. It's updated often and only costs a few dollars at most. *Please* don't steal from this great developer.

Anyway, for this to work, your SD needs 2 partitions. Apps2SD has a partitioning tool, I tried it, it deleted all my files. You can still use the app's partition tool to create 2 partitions, then copy your personal files back when finished. 1st partition is for personal stuff, 2nd partition is for Link2SD/App2SD (I dont recommend storing other files here, but App2SD shouldnt mess with them). I assume most people use Windows, so backup all your SD files to PC, online, wherever, just in case. Download the free version of MiniTool Partition Wizard, resize the first partition so that it's smaller. 1st partition should either be exFAT or FAT32. Then create 2nd partition, format it as ext4. You can use almost any Linux-based filesystem for 2nd partition, but I recommend ext4. Choose whatever size you want for each. You can also use any other partitioning tool of your choice (I prefer GParted, a Linux software). Then put SD back into phone and install App2SD. Make sure you use v11.4 of the app or newer, this is the version that fixed sdcardfs, which is what causes this problem in the first place, and why I started this topic. Tap "link apps to SD card", select ext4 (if this is what you used) and advanced mount option. I have tried the init.d and su.d methods, but after rebooting I got errors like "root access denied", "soft reboot required", and linked apps missing. Advanced method has something to do with Android's vold, is all I know, but works flawlessly for the v10 MM. Apps2SD is better than XInternalSD because you can link all parts of an app (APK, dex/odex, libraries, internal/external data, and caches), XInternalSD only works for external data and obb files.

For Apps2SD, you also need this file:

https://xdaforums.com/attachment.php?attachmentid=3987630&d=1483343643

Unzip the file, force stop Apps2SD, then go to /data/data/in.co.pricealert.apps2sd.pro/app_binary/apps2sd. Delete this file, leave the rest. Then copy the file from the zip into the same folder. For the next part you need to set permissions, I use Total Commander. In TC I hold my finger on the file, select Properties, select RWX, type 777, then Apply. Go to Properties again, select UID, in both fields select the # that says Apps2SD, then Apply again. You can use any root file manager for this, instructions will vary, I just used TC as an example (great and powerful app, BTW). Make sure you do this step. This fix may not be needed in future Apps2SD version, so my advice is dont do this unless obb/external data linking doesnt work or gives an error ("invalid argument"). But this is definitely necessary for v11.4.

If you are still having issues, try these:

In SuperSU, disable mount namespace separation and enable su on boot.

Try setting SELinux to permissive instead of enforcing. For this you can use SELinux Mode Changer, an init.d/su.d script, or you can manually type "su" then "setenforce 0" in a terminal app.

Change your root management app. I used to use SuperSU, but switched to phh's Superuser not too long ago. There is a regular version and a Magisk version, take your pick. I'm loving it and have no intention of going back to SuperSU unless I have serious issues (none so far in the few months I've used it).

XInternalSD only requires Xposed but doesnt need root access, so it may be better for unrooted users. The Xposed framework itself can be installed into system partition with no need for being rooted, but I do believe that the systemless version of Xposed needs SuperSU. The one exception I can think of is the Magisk version of Xposed, which only needs Magisk but not root.

Apps2SD needs root, except maybe for the regular/normal mounting method (haven't tested that). I'm not aware of any solutions for unrooted users, your choices are very limited.

In further testing FolderMount and Obb On SD still do not work with v10 MM. I also dont recommend using adoptable storage for your SD, it's confusing and too much of a headache.