c-old-style-variable-behavior is a variable defined in `cc-vars.el'.
Its value is

*Enables the old style variable behavior when non-nil.

Normally the values of the style variables will override the style
settings specified by the variables `c-default-style' and
`c-style-alist'. However, in CC Mode 5.25 and earlier, it was the
other way around, meaning that changes made to the style variables
from e.g. Customize would not take effect unless special precautions
were taken. That was confusing, especially for novice users.

It's believed that despite this change, the new behavior will still
produce the same results for most old CC Mode configurations, since
all style variables are per default set in a special non-override
state. Set this variable only if your configuration has stopped
working due to this change.