[Magisk] prot0byte:
LV is A9. Is this an A/B device? Ramdisk (in boot) = Yes or No?

Yes it is an A/B device. Yes, it has ramdisk and shows "Yes" in magisk.

Do you mean stock Android upgrade required first?...

I mean going to LineageOS they recommend "Upgrading your firmware before upgrading LineageOS". Here is their documentation https://wiki.lineageos.org/devices/guacamole/fw_update I hope that helps answer your questions

So Magisk root gone entirely? (My guess.)

Correct

So you patched / flashed boot image extracted from current ROM, ie. LOS 18.x (18.1)? Guess you needed a payload dumper tool to extract it? Or were you able to use a TWRP backup?

I did extract it from a payload dumper tool. It's linked in the previous lineageos link I posted for upgrading the firmware. Yes it was LOS 18.1 but OnePlus's flavor of Android 11 which is called OxygenOS. Also, your TWRP question, I have a question in regards to that. LineageOS has their own recovery image. I've been using that because I just figured it was already there. But since my device is no longer booting to recovery, can I try flashing TWRP and use that instead?

I'm guessing you have A/B device w/ a recovery type ramdisk in boot partition (ie. no boot.img in ROM)?... Or is it A-only w/ Magisk installed in Recovery (ie. w/ Recovery mode on)?

Yes it is A/B and had a working recovery, but now I can no longer boot to it. I flashed the boot.img as per the LOS firmware upgrade documentation directed. But then flashed the magisk patched image. I'm not sure I answered your question. I will note as action to try and resolve it, I've uninstalled magisk now. I'm not sure where that leaves the boot.img.

However this is sounding very like you have a mismatched kernel. This is why I'm guessing you have an A/B device and are patching boot.img, not recovery.img, and used wrong image. Since Magisk is actually in recovery's ramdisk (which is in boot partition), recovery may likely also be affected...

Kernel must be matched with ROM. As this resides in boot.img, using (patching/flashing) wrong (mismatched) boot.img will result in many issues, especially comms and other hardware issues.

So did you use unpatched current boot.img, ie. extracted / sourced from current LOS 18 ROM?

Well, the LOS upgrade documentation says to get the latest OnePlusOS (OxygenOS) then extract it and flash many of the images (but not the boot.img), then download the LOS latest zip and sideload it, so I did.

However, when it came to installing magisk, I put the boot.img (the one extracted from OxygenOS using the payload dumper) in my Downloads folder, so that I could use magisk to patch it. Then proceeded to patch it and then flashed it with the produced patched boot image. Which is from OxygenOS not the LineageOS. So I think this answers your question. Which now that you have asked this question and explained to me what's going on, I think this is my mistake.

One last note, in an effort to correct these symptoms, I did flash the unpatched boot.img (i.e. the one extracted from OxygenOS, not LineageOS), so it most defiantly is mis-matched now.

By 'Original', do you mean old LOS 17, later stock, or unpatched LOS 18.x? ... If not LOS 18.x it's wrong (unless you changed OS)...

Nb. When John says 'stock image' in Installation Instruction page, he doesn't account for custom ROMs an really means 'unpatched image'.

No LOS 17 or 18, but the one extracted from the OxygenOS which was downloaded from OnePlus's website and extracted with payload dumper. Yup, pretty sure I see my issue now. I'm a dumb dumb.

So this is confusing; do you actually have a recovery image / partition?

Yes, the recovery image comes with LOS. With LOS 17 I didn't have to do anything. But since I upgraded and re-installed magisk wrong (apparently), I tried manually re-flashing the LOS recovery image from their download pages and it still won't boot to recovery.

If you flashed stock boot.img over custom LOS, that would make sense...

Well, I think that's a "Yes", but just for clarification, I didn't originally flash the boot.img over custom LOS. I guess I did indirectly from "patching" it with magisk and then flashing that patched image. And then again with the unpatched one in an effort to try and fix these issues.

So you may restore by flashing LOS 18.x boot.img (assuming it's A/B and LOS 18.x OS is current)... But you may need to reflash stock A11 ROM first as you indicated was an initial requirement... Also, I'm not experienced w/ A/B devices...

For clarification I never unzipped the LOS 18 zip because the documentation says to sideload the zip. So I just unzipped and extracted the payload and sure enough, it has a boot.img.

In closing, thank you for your time PW. You asked all very good questions, questions which enlightened me and made me realize my mistake before I even answered them. I hope my replies where clear enough. I'll report back once I have time to get back to trying to correct this.

Cheers