Yep, that is already implemented and it can be extended to also check for certificates expiration in the chain, even though that is a more common case for big corporations/governments where they have custom chains of certificates. The notification part is yet to be implemented, but that is trivial.
Technologically I know it’s easy, I think certbot even features a daemon that does this automatically these days.
The main concern I want to point is that this is as @Hermanverschooten said, a negative evolution. If we talk about operational costs, how much it costs to keep a server that sends those expiry emails? 10$/month? make it 100$. We as developers know that development of such a platform is a few days at most of solo work.
I fear that this is just a begging to letsencrypt vanishing from the map forever. This will also sign to the fact that internet will be in full control of corporations, which is not something I look forward to.
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