As mentioned above, some scope names in syntaxes were changed and made more consistent over different languages. This included a change from
keyword.declaration.*. To preserve the original highlighting style in color schemes for those keywords, new ST builds use the style for the
storage.type scopes, if there is a rule for those in the color scheme, but no rule for
keyword.declaration. This is the case for Monokai.
Now here comes the culprit: Ruby seems to have used
keyword.control.* for these keywords in ST3, so unlike in most other languages they were already red in Monokai.
A similar thing happens for the CSS units, which used
keyword.other.unit before, but are now
You should use the following customization for Monokai to restore the old styles:
"scope": "source.ruby keyword.declaration, constant.numeric.suffix.css",
If you want red declaration keywords for other syntaxes too (unlike in ST3), you could add the scopes
keyword.declaration.function, keyword.declaration.class to the scope selector.