You say this is a regression. Can you point to a version that worked as you want? The IPv6 URL handling you pointed out seems to have been added to 2.4b1, and then backported to 2.3.5. I expect that earlier versions didn’t support IPv6 URLs at all.

Also, it does not seem to me that parsing the square bracket form was intended if the port is specified separately. Look at the test cases added in revision 433606e9546c. Perhaps you aren’t encoding the URL correctly, or Requests isn’t parsing the hostname correctly.

Also, according to RFC 6874, it would be more proper to encode the percent sign as %25, so the URL would have [fe80::26a9:37ff:fe00:f764%25eth0]. But I don’t think Python supports this; see Issue 23448.