We are currently using SDL Web 8.5 and planning to upgrade to SIte 9.1. With Web 8.5, we are using Tridion 2013 deployer model (Http Upload). So is it possible to use the same deployer model in Site 9.1?
As far as I can say - It's unsupported - It's not been tested by the Engineering team and you are taking the risk.
Why you need Tridion 2013sp1 HTTP upload in sites 9.1?
The latest version of deployer services has been changed a lot and improved, since sites 9.0 onwards introduced to support multi namespace for both SDL Tridion Sites and SDL Tridion Docs, The Content Delivery deployer it comes with different favors, dx, sites, docs, deployer-combined, deployer worker and deployer services.
Rolling upgrades are only supported and tested from the previous point release.
From 8.5 to 9.1 you'll have to use a staged upgrade route to 9.1.
I hope it helps.