The bug

Prior to Beta 1.9 Prerelease 6, nether portal blocks were affected by smooth lighting like other blocks were, resulting in lit nether portals gradually looking brighter toward the center. However, as of Beta 1.9 Prerelease 6, this no longer happens (potentially due to MC-50734), resulting in nether portal blocks having constant shading. This behaviour persists to the present day.

Despite this change happening over ten whole years ago, I am reporting this as a bug due to this change being completely unannounced from what I can see.

How to reproduce

1. Light a nether portal with smooth lighting enabled, and preferably at a low brightness setting
2. Copy that portal's frame an extra block outwards as to accentuate any shading on the portal
3. Note the lack of any darkness appearing on the portal blocks
4. Repeat this in version Beta 1.9 Prerelease 5 or earlier

Expected results

Due to smooth lighting, the nether portal would appear darker.

Actual results

This is not the case in modern versions; the expected effect can only be seen in much older versions.