We are planning to upgrade Tridion 2013 SP1 HR1 with Razor Mediator to Tridion Sites 8.5. Are there any breaking changes in 8.5 for Razor Mediator? Will Razor Mediator be supported in Tridion 9?
The Razor Mediator is and community built extension and thus is not supported by SDL. However it is using the Modular Templating Framework and its supported extension points, so from that perspective, I would expect the Razor Mediator to remain to work. If not you probably want to contact its owner and let him know.
As for the extension points being supported in Sites 9, since the Modular Templating Framework is not deprecated in Web 8.5, it will for sure still be there in Sites 9, so also there I would expect the Razor Mediator to still work.