I do that on several of my hobby nodes. I block entire ASN's for all the major platforms. Real people can still reach them just fine. To your point I do less of that on my self hosted mail servers and instead use a regex methodology called S25R created by a mail admin in Japan a long time ago and it works great.
Tricky thing about Google is quite a lot of my contacts are on Gmail or some domain hosted by Gmail so blocking Google's ASN is a no go for me. I'm now with Fastmail -- they use Spamassassin (plus I suspect their own custom rules) which uses a range of different metrics to determine whether an email is spam. That is is far more effective than straight up blocking ASNs and the like.
