Prisma does subscriptions and is also based on Apollo server. Prisma supports every database Strapi does and more, so I don't see why Strapi doesn't support subscriptions or why it may be so difficult.

It is pretty difficult keep sync information using relational databases as a main database, it can be a project with the same size that strapi/core is right now, i think that it will never comes, or at least 3 or more years when all strapi use cases are covered.

Do you mean that there is no problem "in theory" but that nevertheless it is currently not supported and not considered on the roadmap, even though it seems to be the second most requested unconsidered developer user experience feature? IMHO, adding GraphQL subscriptions to Strapi would turn it into a Meteor killer.