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Back in the day, there may have been more leniency to delete an account here and there, and some likely DB error/locks may have caused UserId to be skipped:
<…> In the forum's first couple of years, accounts would sometimes be deleted. Also, in rare cases an ID can be skipped (eg. if a database transaction gets rolled back).
In April 2022, the number of deleted profiles (deleted or skipped IDs really)
was 16.117, meaning the number of cases has not increased in a year and a half.
Further back, in
November 2018, I made the figure to be roughly the same (16K), so there really are no account deletions taking place nowadays.
See also
this post.