I think there might be some misunderstanding of what 'long term support' means. It does not mean 'an unchanging lump of code fixed for all time with only grudging security updates'. It means that your vendor undertakes to maintain the software and make whatever changes may be needed to keep it working effectively, while avoiding changes that will stop it working. (These two requirements do conflict a little, since any change at all has the chance of breaking something.)
In this case, the only meaningful way of providing long term support for an antivirus product is to keep it updated with the latest code and the latest virus definitions. To keep it frozen at an old (and therefore ineffective) version is not support at all.