What changed?

Discord changed SlashCommands into Application Commands in favor of the new context menu stuff. You can read about it here.

Upgrade from DisCatSharp.SlashCommands to DisCatSharp.ApplicationCommands

In DisCatSharp.SlashCommands you used Application Commands like this:

// Usage directives
using DisCatSharp.SlashCommands;
using DisCatSharp.SlashCommands.EventArgs;

// Extension
    public static SlashCommandsExtension Slash;

// Injecting SlashCommands
    Client.UseSlashCommands();
    Slash = Client.GetSlashCommands();

// Registration of commands and events
    Slash.SlashCommandErrored += Slash_SlashCommandErrored;
    Slash.SlashCommandExecuted += Slash_SlashCommandExecuted;

    Slash.RegisterCommands<SlashCommands.Main>();

// Events
    public static Task Slash_SlashCommandExecuted(SlashCommandsExtension sender, SlashCommandExecutedEventArgs e)
    {
        Console.WriteLine($"Slash/Info: {e.Context.CommandName}");
        return Task.CompletedTask;
    }

    public static Task Slash_SlashCommandErrored(SlashCommandsExtension sender, SlashCommandErrorEventArgs e)
    {
        Console.WriteLine($"Slash/Error: {e.Exception.Message} | CN: {e.Context.CommandName} | IID: {e.Context.InteractionId}");
        return Task.CompletedTask;
    }

// Commands
using DisCatSharp;
using DisCatSharp.Entities;
using DisCatSharp.SlashCommands;
using DisCatSharp.SlashCommands.Attributes;

namespace TestBot.SlashCommands
{
    internal class Main : SlashCommandModule
    {

        [SlashCommand("test", "A slash command made to test the DisCatSharp library!")]
        public static async Task TestCommand(InteractionContext ctx)
        {
            await ctx.CreateResponseAsync(InteractionResponseType.DeferredChannelMessageWithSource);
            await Task.Delay(5000);
            await ctx.EditResponseAsync(new DiscordWebhookBuilder().WithContent("5 second delay complete!"));
        }
    }
}

In DisCatSharp.ApplicationCommands you use them like this:

// Usage directives
using DisCatSharp.ApplicationCommands;
using DisCatSharp.ApplicationCommands.EventArgs;

// Extension
    public static ApplicationCommandsExtension ApplicationCommands;

// Injecting SlashCommands
    Client.UseApplicationCommands();
    ApplicationCommands = Client.GetApplicationCommands();

// Registration of commands and events
    ApplicationCommands.SlashCommandExecuted += Ac_SlashCommandExecuted;
    ApplicationCommands.SlashCommandErrored += Ac_SlashCommandErrored;
    /* New stuff - context menu */
    ApplicationCommands.ContextMenuExecuted += Ac_ContextMenuExecuted;
    ApplicationCommands.ContextMenuErrored += Ac_ContextMenuErrored;

    ApplicationCommands.RegisterCommands<SlashCommands.Main>();

// Events
    public static Task Ac_SlashCommandExecuted(ApplicationCommandsExtension sender, SlashCommandExecutedEventArgs e)
    {
        Console.WriteLine($"Slash/Info: {e.Context.CommandName}");
        return Task.CompletedTask;
    }

    public static Task Ac_SlashCommandErrored(ApplicationCommandsExtension sender, SlashCommandErrorEventArgs e)
    {
        Console.WriteLine($"Slash/Error: {e.Exception.Message} | CN: {e.Context.CommandName} | IID: {e.Context.InteractionId}");
        return Task.CompletedTask;
    }

    public static Task Ac_ContextMenuExecuted(ApplicationCommandsExtension sender, ContextMenuExecutedEventArgs e)
    {
        Console.WriteLine($"Slash/Info: {e.Context.CommandName}");
        return Task.CompletedTask;
    }

    public static Task Ac_ContextMenuErrored(ApplicationCommandsExtension sender, ContextMenuErrorEventArgs e)
    {
        Console.WriteLine($"Slash/Error: {e.Exception.Message} | CN: {e.Context.CommandName} | IID: {e.Context.InteractionId}");
        return Task.CompletedTask;
    }

// Commands
using DisCatSharp;
using DisCatSharp.Entities;
using DisCatSharp.Enums;
using DisCatSharp.ApplicationCommands;
using DisCatSharp.ApplicationCommands.Attributes;

namespace TestBot.ApplicationCommands
{
    internal class Main : ApplicationCommandsModule
    {

        [SlashCommand("test", "A slash command made to test the DisCatSharp library!")]
        public static async Task TestCommand(InteractionContext ctx)
        {
            await ctx.CreateResponseAsync(InteractionResponseType.DeferredChannelMessageWithSource);
            await Task.Delay(5000);
            await ctx.EditResponseAsync(new DiscordWebhookBuilder().WithContent("5 second delay complete!"));
        }
    }
}