Since 1.15, the accessibility icon (texture\gui\accessibility.png) does not support semi-transparent pixel (even if normal pixels and transparent are normally supported).
Demonstration with screenshots:
1. First attachment, a Photoshop screenshot where you can see that i replaced the selected version of the accessibility icon by a square of pink pixel (so that it is visible) with 50% transparency.
2. Second attachment, a Minecraft screenshot in 1.14.4 where you can see that with the file created with the first attachment, you can see behind the icon when it is selected (normal with 50% transparency).
3. Third attachment, a Minecraft screenshot in the last snapshot (20w27a) where you can see that this time, when you select the icon, you can't see behind it and it's like the pixels aren't transparent (which is a bug because they are.).
How to reproduce:
1. Install the resource pack uploaded in attachment
2. You can see that with the same pack, the icon is not the same (when selected) between 1.14 and 1.15.x, 1.16.x...
I think that others problems have been reported for the same problem on other files.