From MDL-30166:

About the "pageheaderconfigablock" string, it seems that was used initially by blocks, but with MDL-19010 (e03c0c1d49dad7f1d7ccaecee33dfde324f3027f) it became unused since 2.0, but was never deleted.

So surely it would be interesting to get rid of it. Adding Helen here to know/consider about that.
(we can add easily one commit to this instead of creating a full new issue, as you prefer).

So, finally delegated to this issue. Ciao