My Minecraft server runs 24/7 at mc.lakuna.pw.
It is possible to connect with a vanilla client.
The online map can be found here.
The server will usually be running the latest version of Minecraft, but you can check the server's status here to make sure.
The server is hosted by Revivenode.
Although it is possible to connect to the server with a vanilla client, I recommend using Fabric with JellySquid's performance mods (Lithium and Sodium). If you use shaders, Iris is a great alternative to OptiFine that works with Sodium.