Prepared statements assume text type in PG10

The test suite for the Go PostgreSQL driver pgx
(https://github.com/jackc/pgx) found an unexpected behavior change in
PG10. Previously, it was impossible to prepare a statement with a
unknown or ambiguous parameter type.

Pre-version 10:

jack=# prepare ps as select $1;
ERROR:  could not determine data type of parameter $1

But on PG10 the type defaults to text:

jack=# prepare ps as select $1;
PREPARE
Time: 0.183 ms
jack=# execute ps('Hello, there');
   ?column?
--------------
 Hello, there
(1 row)

Time: 0.437 ms

I looked through the git log and couldn't find any commits referencing
this. Is this an intended behavior change?

Jack