I think they already reach the traffic bandwidth for this month that's why preev is down.
According to
heroku limits "Network bandwidth must not exceed 2TB per month." so the website already reaches the 2TB this month that cause this issue.
Similarweb showing that they have more than 1 million traffic monthly but since similarweb has 3 months delayed I'm sure that they got more than that today due to the price increase.
So this might be the reason why preev is down.
Anyway, I would like to share this site below as an alternative that looks similar to preev.
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https://bitcoinpreev.com/and this one as an alternative
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https://currencio.co/
It's also being discussed here
https://bitcointalk.org/index.php?topic=5290493.0