In my searching around about beach volleyball on various sports related websites I've found these and am posting the links in case they are of use.
The IOC has a webpage listing minimum ages for athletes. These ages are apparently set by each participating country. GB (and many other countries) are 13+. France is 15+. The Netherlands is 16+. How would France and the Netherlands plan on supervising a known sex offender who might be around such young athletes? Would the managers of other country's teams with under-age competitors be told about him (or any others, he might not be the only one)? Presumably all of the athletes will have access to shared spaces within the athletes' village eg. training and catering facilities.
https://olympics.com/en/age-consent
In sports that I'm more familiar with, UK teams have nominated welfare officers and supervision should be high, although it still has the potential to fail. It may not be like that for other countries, even ones that look fine on the surface, eg. Nassar et al being 'hidden' in US gymnastics.
The International Volleyball Federation (FIVB) seems to be the regulatory body responsible for all forms of volleyball worldwide. They maintain a database of the results of all relevant international competitions and it's possible to search it for individual players. This is Steven Van de Velde's page: https://www.fivb.com/players/players-database/player/138207
It shows that he was competing regularly on the international stage up to 28th Aug 2015, then there was a gap until May 13th 2017 when he resumed. He's then competed regularly ever since, including at European and World Championships.
Why hasn't there been any publicity about him in the sports' world before now?