Borrowing the syntax from POSTGIS, "SRID=<number>;<WKT>"
The release notes for 19.x should contain a mention of this improvement. From the updated ECQL docs:
Starting with GeoTools 19.0 Geometry objects carrying a CoordinateReferenceSystem among user data get encoded as EWKT. If you do not desire so, set the Hints.ENCODE_EWKT system hint to false (e..g, Hints.putSystemDefault(Hints.ENCODE_EWKT, false);).