There aren't any known vulnerabilities in XenForo. Unlike some other forum systems that release security updates every few months, XF's code is extremely well written, minimizing possibility of any security issues.
Although, I'm not all that certain the OP was talking about security risks:
Not knowing does not mean there is no security risk, it just means that the security risk isn't known / popularly known yet
It would be unwise to assume there are no security risks in any software (even if we are just talking about high priority/severity security risks)
The chances are, if there are no future upgrades and security risks are found... If there are still XenForo boards around, some one will design a plug-in to fix the risk (I certainly would).
But, I agree... Not upgrading any piece of software does leave you open to future security risks if and when found
As it stands XenForo is an amazing piece of forum software, the core is great (and since it's well written, it can be taken very far with plug-ins / easily customised)
I think kembaraHadi is correct to consider this point, and it should be considered when evaluating software, but I don't think it is correct to say there are no risks (even if it is an opinion)
If you are just asking what are the chances that the software will no longer be updated, there are very few people that really know the answer to that, I suggest following these:
It seems you are using an unlicensed forum. XenForo has a very neat code and it's best you support them. If you can't afford it, try opting for a free option, until you earn the money - otherwise you could run yourself into some troubles. Cracked/nulled software can be loaded with viruses, bugs and security exploits and back doors. Just be careful what you get yourself in to.