Warning: DO NOT try to silence the nagging by blocking the notifications. AT&T/LG will treat "quit bothering me" like "go ahead, have your way", and simply go ahead and try to install the update without even bothering to ask you next time. If you're running RootedStock, you'll end up in a slow semi-bootloop where Android boots, you'll have about 3-5 minutes, then it will forcibly install the update, reboot, you'll end up in TWRP, and when you reboot to system (or power off, then power up later), the whole cycle will repeat.
I'm not sure how robustly my solution worked, but here's how I broke out of it. So far, about 15 minutes after rebooting, it seems to have worked. Since I'm writing this AFTER seemingly fixing the problem, I'm writing it from memory, and can't say with 100% certainty which step was actually the one that fixed the problem for me. I actually began the process of installing Titanium backup about a half-dozen FOTA-reboot cycles before tripping over another post here that advised deleting /cache/FOTA. Titanium Backup might, or might not, be an essential element of my apparent success. I honestly don't know. If you own it, use it... you have nothing to lose. If you don't already own it, try just deleting /cache/FOTA first from TWRP's file manager and see whether it works (temporarily or permanently).
Anyway... here's the approximate path to slaying the FOTA beast:
1. I installed Titanium Backup from Google Play. It actually took two reboot cycles to do this... you REALLY have to be ready to unlock the phone, launch Google Play, search for Titanium, and trigger its installation while standing a few feet from your wifi AP for it to finish the download and begin installing it before the next forced-update attempt begins. When it does, swear violently. It'll make you feel a tiny bit better.
1b. When FOTA forces the reboot into TWRP, try deleting /cache/FOTA before rebooting. It might work to temporarily slay the FOTA beast, it might not. If it does, it'll save you about 30 minutes of FOTA-reboot misery for the remaining steps. Feel free to repeat this after each of the following steps. It can't hurt.
2. On the next cycle, launch Google Play the moment your phone finishes booting, and go back to Titanium Backup. Hopefully, it'll be installing. With a little luck, installation will finish before FOTA begins. Then sigh, and let FOTA waste another 5 minutes of your time.
3. On the next cycle, launch Ti backup, give it permanent root permission, and give it permission to do everything it wants. Then, try to launch Google Play, search for Titanium, scroll down, select Ti Backup pro key, and try to initiate installation before FOTA kicks in yet again.
4. On the next cycle, launch Google Play, search for Titanium, scroll down, select the pro key, and begin installation if it isn't already downloading and/or installing. 99% likely you'll end up going through another round of FOTA misery.
4b. If FOTA is still forcing reboots up to this point, repeat step 1b before step 5.
5. On the next cycle, launch Titanium Backup, give it any additional permissions it wants, then select Backup/Restore, scroll down to "FOTA Update 8.0.0", and freeze it.
5b. If FOTA kicks in again, repeat step 1b.
If, despite deleting /cache/FOTA, then freezing FOTA Update 8.0.0 using Ti backup, then deleting /cache/FOTA again, it's still happening... well, then my solution didn't work for you (and possibly not for me. I'll start breathing again normally if my phone is still working normally tomorrow).
Anyway, hopefully this will help someone. I suspect a lot of people running RootedStock Oreo are going to get stung by this. I'd guess that more than a few v20 owners actually bought theirs long after AT&T's previous forced update, and have never actually HAD to deal with blocking forced updates with their current ROM.
It's entirely possible that purchasing Ti backup (I don't think the free version can freeze apps) will be essential for the H910 going forward if you want to keep using what was, prior to yesterday, the newest rooted-stock Oreo ROM without having AT&T's FOTA harass you every few hours (or wipe your phone and reflash once someone releases a newer build based on the current update). Trust me, you can't swat it away forever. I went through this forced-update nagging bullshit years ago with my Motorola Photon. Someday, when you least expect it, the dialog will come flying at you when you're driving and trying to select a song using Amazon Music, or in the middle of a phone call, or semi-distracted, or scrolling down a web page while the phone is bogged down because it's grinding its wheels in the background preparing to harass you about updating again, and you WILL accidentally click "ok" & have your life go down the toilet for the next few hours until you're able to dig yourself out of the mess.
Bite the bullet, and freeze FOTA Update 8.0.0 now, while you can still do it without burning an hour of your life waiting for reboot after reboot.
