Windows XP became officially unsupported by Microsoft in 2014, and Vista in 2017, which means that any security vulnerabilities found since then have not resulted in any fixes to XP or Vista, leaving users' machines at high risk of attack with very little defence - even anti-virus cannot stop many common exploits of operating system weaknesses.
Further to this, SimSig License Manager requires a secure connection between it and the SimSig Website in order to protect users' information and licenses. Windows XP and Vista does not support the more recent ciphers and protocols that all modern websites should be using. While some web servers can fall back to older standards, doing so opens up that server to known security vulnerabilities. For that reason SimSig will not, and legally and morally should not, support the older standards that XP and Vista support which have been identified as unsafe. This is for the protection of all our users.
We understand that a tiny number of users may still be on XP and/or Vista. If users are unwilling or unable to upgrade to a more recent version of Windows then we sadly cannot continue to serve those users as customers. The safety of the majority will not be compromised by the actions of a few. This is a limitation of XP and Vista, not a limitation of SimSig.
Vista does have possible workarounds, but XP does not. One can search for them on the Internet but we're not linking to any here as it does not solve the underlying problem of those operating systems still being vulnerable.
While no more recent Windows versions appear to be affected yet, be aware that any that become unsupported by Microsoft may also stop working. SimSig cannot guarantee that any obsolete operating systems will be supported.