This graph appears to show that most people polled (97%) are using MySQL 4.1 or later
This second poll shows, again, that most people (89%) are using 4.1 or later.
The thread at the PunBB forum gives 95% of people polled using 4.1 or later.
The Mambo forum poll from 2006 gives 80% using versions after 4.1 (with 8.5% don’t know or other)
Admittedly not scientific surveys (self selection, etc) but somewhat convincing.
This has an obvious implication on how to handle MySQL support, in particular special casing to work around particular issues in early versions. This code adds complexity and in some unfortunate cases also bugs. The less there is, the better.