When making games or fanmade content, people often discuss the topic of difficulty balancing, or the "good type of difficulty", or when something is "too difficult", but honestly there's no very specific answer to that, because to some extent it's gonna be subjective, even on the topic on whether something is fair or not.
One very important thing to keep track in your works is to make sure your challenge has a proper curve, so that players can slowly warm up to it, and make sure to know your target audience. There's a good reason why commercial games usually try to be hard, while still giving the player a lot of room for error. If a game is way too unforgiving, or too inaccessible, few people are going to buy it. That's why the absolutely hardest stuff is very likely gonna be fanmade content, and not a product that you'll purchase on Steam or somewhere else, or they will have built-in accessibility features to lower the difficulty if needed.
As a creator it's easy to get carried away and to accidentally make something way too tough, hence the whole "creator bias" or "creator goggles" terms are often used. Through creating and extensive playtesting, you develop muscle memory which makes you easily forget on how hard something actually is, and that's why playtesters are a very important part of your designing process, as they are people who aren't gonna know much as they start out, and they will most likely have a much harder time going through your game, as they're not familiar with it yet. With enough experience as a designer or modder, it's possible to get better at judging what's fair for other players, and what's not, but even the best level creators have playtesters for a reason.
There's also the concept of how a level is supposed to be played. Especially with the most brutal stuff out there, sometimes they require others to play them in specific ways to be cleared. Some fanmade content might be blind-friendly, but some may be built with pre-practicing in mind. What's pre-practicing? It's using mid-level checkpoints that help players to memorize or figure out fragments of a stage, in order to be able to reasonably beat them. This may be done in form of savestates, or optional checkpoints or practice modes (something like what Geometry Dash does).
It's not always obvious on how the author has intended their work to be played, but it's something worth keeping in mind, as I've seen players attempt levels that weren't built to be blind-friendly without any pre-practicing tools, and eventually end up being extremely frustrated or discouraged to play. Using that philosophy is also not inherently bad level design, but it's definitely nice if the player instantly knows what they're getting into. At some level of challenge, when you reach the high-end, it may even be a necessary thing to be able to complete something within a reasonable timeframe.
This design thought was partially some advice, but also a mini ramble on how I've seen people approach hard levels, and struggle to understand that there's multiple ways to play or learn something. I understand that it's not easy to adapt to a different playstyle, when you've been clearing games or custom content in a specific way, but it's worth knowing that it's not necessarily bad design, but more of a niche that will not work for everyone. I'm here speaking from the perspective of Celeste mods, but this might as well apply to a lot of other games.
