Per challenge of updating OC some tips:
so have finally got the system back to a working BigSur 11.4 on OC 0.7.0 via SSD EFI.
• ocvalidate does-not and can-not ensure that a config.plist is correct/useful. It's a syntax checker that helps you eliminate grammatical errors.
• ocvalidare can tell you when config items are needed-but-missing, deprecated and mis-specified. It does sanity checking of value type/format per the spec, but can't know if config is correct for a use-case.
• Every OC release includes documentation (spec) Configuration.pdf which explains every config schema item, with the accompanying sample.plist to show examples of each.
• The most common errors are naturally old config.plist items which have been removed from spec (unknown config that's no longer a part of OC or user error) and new OC schema additions where your old config.plist needs addition of parameters or changes of param values to accommodate new/changed features. The bigger the version difference, the more parameter / schema changes you can expect.
• Schema means the format and range of acceptable config.plist entries. IOW ocvalidate compares every entry it finds with the schema for that OC version to determine if the entry fits in context. When ocvalidate reports "no schema" its saying it doesn't recognize the entry in context. Just because it doesn't recognize an entry doesn't make it wrong or bad. Ocvalidate doesn't know anything about appropriateness of config. It's just a warning to you to remind you to understand the entry and why it's there.
• When ocvalidate calls out key errors, you need to fix those because the given entry will not function as intended if it's parameters are incorrect. "failsafe" is the default value which OC adopts when it encounters parameters it doesn't recognize.
• The job of creating a correct / useful config.plist is far beyond the scope of ocvalidate. Configurators offer more per-build contextual assistance, with the caveat that they also add additional ways to be wrong, including that the configurator may produce config which is erroneous for a given version of OC, which ocvalidate may or may not detect.
In general, you should attend to all ocvalidate errors because they suggest likely configuration errors, ranging from an incomplete config to wrongly-specified config where at its worst, the config.plist is so defective that OC cannot completely interpret it. For this reason any error from ocvalidate should be considered unacceptable even though an erroneous config.plist could can be found to work.
Consult the Configuration.pdf that comes with the OC version to understand OC parameters and formats and gain insights into features.
In your case, avoid configurators
as they will lead you down a primrose path which might just as well introduce errors for your old build as help you avoid them. As you had a previously working config and as when OC advances it doesn't tend to invalidate previously working configs (notwithstanding that a config.plist could have errors and still work, and that OC progress could expose such an error in a build, but let's assume your config is basically OK) it's safest to do what you have been so far: just run ocvalidate and look up its errors in Configuration.pdf.
For example, per your post in the other thread on 0.8.7 diffs
NOTE: This version of ocvalidate is only compatible with OpenCore version 0.8.7!
OCS: No schema for HibernateSkipsPicker at 2 index, context <Boot>!
This says that ocvalidate doesn't recognize the parameter HibernateSkipsPicker. Is it in documentation Confguration.pdf?
OCS: Failed to calculate size of true field containing <empty> as type integer, context <ShowPicker>!
This says ShowPicker's value is the wrong type or is not appropriate. Again look in the documentation.
Serialisation returns 2 errors!
Completed validating /Volumes/Big Sur — Data/Users/Desktop/OC 0.8.7 Big Sur/plists/OC 0.8.7 Boot Fail.plist in 1 ms. Found 2 issues requiring attention.
I was not able to follow how your USB got impugned in all this re boot failure with prohibited sign—maybe I didn't read thread carefully enough— but if you have boot picker config errors it's not a surprise to see a system respond with a basic OS load fault.