There's an ASP.NET MVC 5 application (it uses Entity Framework 6.0, .NET 4.6.1 dependencies, it utilizes both MVC controllers and API controllers and most importantly) which relies on IIS's ability to provide session stickyness through ARR. Sometimes this is also referred as client affinity and IIS ARR implements it with a cookie if I'm not mistaken. ARR feature is essential for that app, it'd be unusable without it.
I haven't found any concrete evidence if such a project got upgraded to ASP.NET Core 3.1+ (or even .NET 5) then would the ARR still work as before? .NET Core architecturally is very different than .NET MVC 5 and want to prepare in advance for any surprises. Is there any cloud platform besides Azure which could provide equivalent feature what Azure WebApps can provide with IIS + ARR?
BTW I highly disadvise anyone from developing any solution which would rely on sticky session or client affinity. Ideally a webapp request/response code should be stateless in the sense that the requests should be able to be routed randomly to any server in case of a horizontal scale-out.
Sticky session routing can be also costly (I mean money) depending on the provider (fortunately IIS ARR is free) and it can also hurt even load distribution.