I'm not sure what you mean by this, since UDRP cases aren't "solved".
It's unclear whether this statistic is the number of cases filed each month, or the number of cases decided each month. But that doesn't really matter.
UDRP cases can be filed even in the absence of information regarding the identity of the registrant. That's not the issue, and I don't believe there were any claims that WHOIS partly "going dark" would result in fewer cases being filed. If anything, more cases (but not better cases) may be filed (see #1 below).
The issues that are caused by lack of registrant information include the following:
1. The potential complainant won't be able to tell whether the registrant may have a legitimate interest in the domain name, and whether that right predates the complainant's rights. This could result in cases being filed that wouldn't be filed with better information.
2. The potential complainant will have a much tougher time determining whether the registrant engages in a pattern and practice of cybersquatting (since the identity of the registrant will not be known at the time of the complaint).
3. The potential complainant will have a much tougher time determining whether other infringing domain names are owned by the same registrant.
4. It will be more difficult to consolidate cases, since it won't be immediately clear that domains in different are under common control.
Best regards,
Greg