Updated statistics after getting at least 500 sample leaf certs per quarter that are within 120 days of the vanguard (see my earlier comment for definition):
I've added a breakdown of certificates by issuer, as well as the estimated number of certificates by quarter.
My biggest doubts are the GoDaddy certificates and how many they've posted to Certificate Transparency, but whatever.
There were a few certificates with oddball key lengths (2432,3096,8096) that I eliminated. The vast majority have historically been RSA 2048-bit, but that's come down in recent years with increases in RSA-4096 or ECDSA-256.
Since ISRG (Let's Encrypt) and Comodo have dominated in recent years, here's their breakdown:
Looks like Comodo made a big push to ECDSA in 2014, then backed off, then pushed again to ECDSA more recently. ISRG has had a smaller push.