Microvacuum for gist. Question about GISTPageOpaqueData flag

I'm working on microvacuum for gist access method.

Briefly microvacuum includes two steps:

1. When search tells us that the tuple is invisible to all transactions it is marked LP_DEAD and page is marked as "has dead tuples",

2. Then, when insert touches full page which has dead tuples it calls microvacuum instead of splitting page.

You can find a kind of review here [1].

While writing patch, I found strange GISTPageOpaqueData flag - F_TUPLES_DELETED.

Its description looks like it is the same for BTP_HAS_GARBAGE

#define F_TUPLES_DELETED   (1 << 2) /* some tuples on the page are dead */

But it's definitely not the same things. I found only two mentions of this flag. 

Function GistMarkTuplesDeleted sets the flag after dead tuples deletion.

Do anyone need it at all? I found no place where this flag is checked.

Is it correct using of the flag?

I need an advice, what would be better: 

- to add new flag like F_HAS_GARBAGE,

- or to delete all mentions of F_TUPLES_DELETED and use it in gist microvacuum.