This has been true since the introduction of bars.png in 15w31a. The boss bar texture in icons.png has remained unused since then.
I'd recommend just outright splitting both files into individual textures, as has been done for other texture sheets over the years, which should get rid of unused textures like these in the process.
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Can confirm in 20w51a.
Can confirm in 21w05b.
Can confirm in 1.16.5.

Can confirm in 21w17a.

Despite Mojang stating they would never fix this, it did end up getting fixed in 23w31a.

Won't Fix != We will never do this. It just means they're not going to go out of their way to fix it (as a bug), if it changes, good, if not, whatever.
This was not done as part of bug fixing, but rather a change in a seperate action in a design change (to allow separate icon overwriting).
Hi there!
I can confirm.