Let me start with the question, itself.
If you found a recycling triangle (and that's really what it is), it means the sink is made entirely of some type of plastic. Most laundry tubs are plastic; it's a bit unusual for kitchen sinks. There should be a number inside the triangle, which is a code for the type of plastic. If you can read it (or make an impression of it with something like modelling clay to see it better), that would help with answering. Different types of plastic get discolored from different kinds of things, and removing the discoloration could require different methods and materials for different plastics.
A picture would also help in understanding the nature of the discoloration. An additional picture of the underside in the area of the triangle would also help in case the sink is a sandwich, and the bottom is a different material from the inside. Another possibility: kitchen sinks don't typically have an overflow hole like a bathroom sink, but those sinks can be like a sink within a sink or have an outer shell for a channel for the drain.
So that additional information can help get your question answered regardless of which site it's on.
Where to post
The question is already on Lifehacks and has received an answer, although pretty much all of the responses there seem oblivious to the part about plastic. With additional information as noted above, you may be able to get an answer there.
Cleaning is a gray area as far as being on topic on Home Improvement. Cleaning refers to removing a foreign substance, which is different from fixing discoloration. Discoloration can be from damage to the material, itself, so fixing it is more in the nature of a repair, and sinks are on topic. A picture would help with clarifying the nature of the problem, but if you repost here, frame the problem as fixing discoloration rather than cleaning.
A good answer will probably require some knowledge of materials and sinks, which is likely to be more concentrated here than on Lifehacks. Lifehacks will have a higher percentage of answerers who have little formal knowledge of materials, so solutions may be generic or even inappropriate for sinks. Also, the pool of users there may be less likely to recognize and comment on inappropriate answers. A solution needs to consider how it will affect the sink material, surface, etc., because removing the discoloration could potentially create a worse problem. So it would be on-topic here, also, if the question is properly framed and contains enough information to be answerable.
Cross-posting isn't allowed, so the question would need to be closed on Lifehacks. You could do that and repost here, or a moderator could migrate it for you after you improve the question if they thought it was on-topic here and detailed enough to be answerable. Migration would also bring the existing answer (although sinks would be tough to make out of slate, and a composite wouldn't have a recycling number, so I suspect the answer isn't helpful in this case).
If it was my question, I would better trust an answer from Home Improvement on this issue. I would improve the question in two ways and repost here after closing on Lifehacks. One improvement would be adding the information and pictures mentioned above.
The second improvement would be making it more clearly on topic here. Frame the issue as discoloration rather than cleaning. Mention factors and considerations that are relevant to the experts here, like the implications of the materials, usage as a sink (which is different from a countertop, for example), and concern for how the repair method might affect the sink. For example: some methods could deteriorate plastic; if the surface is shiny, and the work leaves it frosted or scratched, it could become harder to keep clean, may encourage growth of bacteria, or may create a spot where stresses concentrate and lead to deterioration; etc.