Indeed, 50% is the maximum permissible value.

What is also confusing is why various plugins read the value as 0.50% instead of 50%

0.50% is vastly different from 50%.

Well, this is not only a documentation issue.  The modifier
callback changes the given value[1], but immediately after return,
the value gets changed back to what was passed[2].  That causes
ini_get() to return the originally set value, instead of the
adjusted value.

> What is also confusing is why various plugins read the value as
> 0.50% instead of 50%

I assume that is because these plugins retrieve the value via
opcache_get_configuration() which returns the value as float
number (not as percentage).  So if the value is given as 40, the
function would return it as 0.4.  The WordPress plugins having it
wrong should just multiply the value by 100 (and maybe round).

opcache.memory_consumption and opcache.max_accelerated_files have
the same problem, namely that they (temporarily) change the value,
but they ignore that change when actually assigning to the member
which holds the value.  So, for instance, setting the memory
consumption to 4 yields a warning that this value is too low, and
that the default of 8 MB would be used, but actually 4MB are used,
causing a fatal error.

[1] <https://github.com/php/php-src/blob/php-7.4.6/ext/opcache/zend_accelerator_module.c#L213>
[2] <https://github.com/php/php-src/blob/php-7.4.6/Zend/zend_ini.c#L254>

> […] but they ignore that change when actually assigning to the
> member which holds the value.

No, these settings are handled exactly like
opcache.max_wasted_percentage.

The current behavior is documented now:
<http://svn.php.net/viewvc?view=revision&revision=350001>

I've filed a separate ticket (bug #79665) for the bug, and close this as doc problem.