First, end-to-end decryption offered by PGP and S/MIME and hop-by-hop encryption provided by SMTP STARTTLS and the TLS use in IMAP and POP are totally different things. With end-to-end encryption the mail is encrypted by the sender and decrypted by the final recipient. With hop-by-hop encryption instead the mail is only encrypted during transit from one mail server to the next, which means it is accessible in plain on each mail server involved and it is also stored in plain at the providers mail server where it then gets retrieved using POP or IMAP.
That said, there are some kind of recent statistics about how much STARTTLS for SMTP is supported in practice:
While these statistics describe how much mail servers support receiving mail bei TLS they don't say how much mail is actually delivered by TLS. There are some interesting statistics from Google which describe how much TLS is used for incoming and outgoing mails from them. It shows for example that at the beginning of 2016 about 60% of incoming traffic was using TLS, beginning of 2017 already 80% and now about 92%. The current number for outgoing mails is around 88%. The last number means that for 12% of the recipients their mail server does not support TLS.
To get some real-life statistics of my own I had a look of some mails I have collected within the last 3 years, about 300.000 non-spam mails and about the same number of spam mails. From the non-spam mails around 77% where delivered with TLS while with spam mails this number was 42%. When only looking at the mails from this year 87% of incoming non-spam mails used TLS while only 53% of spam mails.