Updated to exclude parameters from being lowercased (thanks to @jonorogers)

LowercaseDocumentFilter.cs

using System.Collections.Generic;
using System.Linq;
using Swashbuckle.AspNetCore.Swagger;
using Swashbuckle.AspNetCore.SwaggerGen;

namespace Heartwork.Api.Application.Filters
{
    public class LowercaseDocumentFilter : IDocumentFilter
    {
        public void Apply(SwaggerDocument swaggerDoc, DocumentFilterContext context)
        {
            // Lowercase all routes, for Swagger, as discussed here:
            //   https://github.com/domaindrivendev/Swashbuckle/issues/834
            // Issue has a reference to the original gist, which can be found here:
            //   https://gist.github.com/smaglio81/e57a8bdf0541933d7004665a85a7b198

            var originalPaths = swaggerDoc.Paths;
            
            //	generate the new keys
            var newPaths = new Dictionary<string, PathItem>();
            var removeKeys = new List<string>();
            foreach (var path in originalPaths)
            {
                var newKey = LowercaseEverythingButParameters(path.Key);
                if (newKey != path.Key)
                {
                    removeKeys.Add(path.Key);
                    newPaths.Add(newKey, path.Value);
                }
            }

            //	add the new keys
            foreach (var path in newPaths)
                swaggerDoc.Paths.Add(path.Key, path.Value);

            //	remove the old keys
            foreach (var key in removeKeys)
                swaggerDoc.Paths.Remove(key);
        }

        private static string LowercaseEverythingButParameters(string key)
        {
            return string.Join('/', key.Split('/')
                .Select(x => x.Contains("{") 
                    ? x 
                    : x.ToLower()));
        }
    }
}