The number of groups found deleted each month has hit an all time low. There’s been a belief that Yahoo! Takes pleasure in deleting large numbers of groups. Personally, I never believed that to be the case. It's been my contention that a bug in the Yahoo software caused many groups to be "lost."
The figure below shows the one of the statistics about deleted groups that I've compiled. The chart below shows about six+ years worth of data.
The bars show the percentage of deleted groups (relative to the number of groups checked) found each month. It can clearly seen that the number has dropped from 4-6% to less than 2%. That 2% figure still represents 500-1000 groups I've found. It appears that the bug has largely been fixed.
In balance though, there is a very large number of groups that are dead or spammed to death that really should be deleted. If anybody at Yahoo wants a list ... let me know.