[Google Pixel 2 XL Questions & Answers] 130rne:
I'm on 10 with an unlocked bootloader, 1st gen Pixel. I still get security updates. Twrp doesn't mount /system or /vendor, so I take system/vendor IMAGE backups with no issue. Not sure if I'll get updates still after unrooting. I've always flashed factory after rooting. Theoretically it should return to stock if you flash the Magisk uninstaller. The boot image (boot + recovery + base system) will be restored. You should then be able to get OTA updates. They will be installed to the OTHER slot, so if the phone fails to boot (3 times?) it will switch back to the first slot. If you have issues and want to go back, you can manually switch slots in fastboot or go into twrp, tap reboot, tap the other slot, then reboot system. It will show a message that the slot has changed. You can of course always check the active slot in fastboot to make sure. If you want to roll back an OTA update, reboot to twrp, change the active slot, then reboot system, and it should boot to the older version. Make sure to turn off automatic updates or else it will redownload the same update again. I believe I've lost twrp by installing an OTA update, which installs to the inactive slot, which overwrites recovery, and then boots to that slot. Meaning if you don't have a pc to switch slots in fastboot, you can't go back to the first slot which still has twrp. The command is fastboot set_active, something like that. Quick google search.. No pc means no restore if something goes wrong while you're away from a computer. The only way I know of to change slots at that point is to hard reset 3 times, which will trigger the phone to switch slots. ALWAYS reboot after flashing bootloader/radio. I'm decently sure that the bootloader has to match the system, so if you install lineage Pie, you need to flash the bootloader/radio from Pie or the phone won't boot. Flash bootloader, reboot, flash radio, reboot, flash matching rom. It is technically possible to have dual boot, where one slot is lineage and the other is stock, as long as they're the same Android versions. Lineage Pie/stock Pie for example, because the bootloader won't match if you flash Lineage Oreo/stock Pie. Keep in mind OTAs will overwrite twrp with stock recovery, so if you reboot, you might lose access to twrp, and I don't know of a way to OTA and reboot on the same slot. I don't know if there is an app or command for this, WHICH WOULD BE REALLY FREAKING NICE. Also, when restoring boot from twrp, I always reflash twrp to make sure. It will install to both slots. You can keep a copy of twrp on your data partition. To reflash twrp from within twrp, Install > Select Image > find twrp.img > flash to RAMDISK. If you flash to boot, it will overwrite system, and leave you with only twrp. When installing in older twrp's, I believe it will install to the same slot. With newer twrp's that specifically support Treble, it will flash to the other slot. Don't quote me on that. Due to the single data partition, there's a chance Lineage will have issues with stock data. I've never seen it though. I've dirty flashed a couple times with few issues. I think I've even dirty flashed different OS versions. If you get app crashes, open that app's info, delete its storage/cache, and reopen the app. Most of the app data is the same across versions anyway. The Pixel is a complete pain in the ass when coming from non Treble phones. Sorry if this is repeated info, the posts above are LONG lol. Hope this helps.

Edit- not sure if relevant to you, but I get bootloops when flashing twrp to the ramdisk. There should be an option in twrp to fix it. I always do this after flashing twrp this way.