Startup Graveyard might be a bit more popular than Five Second Test. We know about 1 link to it since March 2021 and only 1 link to Five Second Test. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
I usually try to vet the idea with others before moving forward. Bounce ideas off other people. Reach out to as many people as possible and ask them if it's a good idea. Some websites like fivesecondtest.com will let you get feedback before the idea is even launched. Source: over 2 years ago
Not subreddit but this one does the trick: https://startupgraveyard.io/. Source: over 2 years ago
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The Google Cemetery - A list of dead Google products and why they died
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Google Graveyard by SaaSHub - The Google Graveyard is the complete list of discontinued products by Google. Also known as 'The Google Cemetery'