In my opinion, the question "What is a glitch and what is a cheat?" is not as important as "What sorts of glitches should we support in questions?", but I will give my opinion on both these topics.
What is the line between a glitch and a cheat?
Naively, a cheat is something that was added intentionally by the developer, in one way or another. If a developer sat down and said "Alright, so if a player enters 🡅🡅🡇🡇🡄🡆🡄🡆, they will have infinite lives", then this is clearly an intentional feature of the game, even though it may be hidden. Other "cheats" may be a reward for completing certain optional tasks.
A glitch is something that should not happen. A very good example for this is the old gravel trick in Minecraft, in which partially submerging yourself in gravel would allow you to see through solid blocks and find caves easily. This feature was clearly not intended, and can thus be considered a glitch.
The trickier question is, when is something a "glitch", and when is something "a very advanced technique"? The skiing example, as noted above, really blurs the line. It's difficult to say, and topic of extensive debate among many speedrun communities. I would argue that defining a clear line when something is a glitch or not is a very philosophical question, which I am not equipped to answer.
Should all cheats and glitches be considered off-topic?
I would say it depends if it negatively affects the enjoyment of others. It's actually a remarkably similar question as to what to do with spoilers for movies or shows, where some hardliners even consider "Wow, that movie was really good" to be a spoiler.
My suggestion is the following: If the execution of a cheat or glitch does not negatively affect the way others can enjoy the game, then it should be allowed. A question like "What are all known cheat codes for GTA: San Andreas?" should not be voted off-topic, as cheating in this game is the sole reason why many people bought it in the first place.
Of course, questions such as "How can I duplicate items on a multiplayer server?" are a gray area. It depends on the people playing there, and if they appreciate item duplication or not. It might interfere with their enjoyment, if one player suddenly has an excessive amount of a very rare item.
Other questions, such as "How can I make an undetectable aimbot?", are blatantly off-topic, as they are bound to be used in a way that negatively affects other players.