SSL is (was?) required. I left Heroku about a year ago and it's nearly inconceivable that this would be changed, having been the case for many years.
I don't think they've implemented certificate validation since I've left though.
My naive hope, going on many years, is that SCRAM with channel binding would have landed years ago (the first versions of the patch began to show up then), making client-side certificate checking (and let's get real: it's hard enough to use that many people will not validate when developing from their laptops, simply backspacing out the optional cert validation connection option, a elision that is invisible to the server) obsolete. It should be possible to modify the definitions of pg_hba.conf to require a channel-bound SCRAM connection, which would mean that the client is certain to have checked for an untampered certificate.
This implementation of SCRAM doesn't have that yet, but it's been an ambition of the author for some time to do so.
A patch implementing channel binding has been presented for integration into Postgres 11: https://commitfest.postgresql.org/14/1153/. Two channel types are presented: tls-finish and endpoint. Per the RFC 5802, it is mandatory to use SSL if you want channel binding as the data needed for binding validation is either the TLS finish message which can be found after the SSL handshake between the server and the client (which happens before the password-based authentication), and a hash of the server certificate. All those things are actually supported by a set of APIs in OpenSSL.
True. I am not saying it's the best idea around, only that it's low friction. I'd probably approach it differently but I can see why they did it like they did.