This was discussed over and over, including here on HN in 2019. [1]
java.net.URL was introduced in Java 1.0, the very first public release of Java back in 1996.
In 2001, Java 1.4 included a new class java.net.URI that addressed this and other improvements.
Since then, it is well known by average experienced Java developers to a) read javadocs, b) not use URL class, and c) use URI.
It's been 19 years this problem has been fixed.
As to why not remove it, I'd suggest to dig through the OpenJDK mailing list.