I believe only Civ2 had celts as starting civ. They also don't fit the alphabetic icons order we have. So yep, most likely they don't come in base game this time either, but they are very highly probable to come later
Given that the Celts have appeared in some form since Civ 2, I wouldn't worry about getting a Celtic civ at some point.
The Civ franchise really doesn't *cut* civs very often. They may get replaced or refined with an equivalent, but by my record:
* Civ 1 -> 2: Nothing
* Civ 2 -> 3: Nothing (Sioux replaced with "Native American")
* Civ 3 -> 4:
Hittites (Iroquois replaced with "Native American," Austria replaced with HRE)
* Civ 4 -> 5: Nothing (Khmer replaced with Indonesia, HRE replaced with Austria, Mali replaced with Songhai, Native Americans replaced with Iroquois, Sumeria replaced by Assyria, Viking replaced with Denmark)
* Civ 5 -> 6:
Morocco and
Venice (Austria replaced with Hungary, Denmark replaced with Norway, Polynesia replaced with Maori, Celts replaced with Gaul, Huns replaced by Scythia, Carthage replaced by Phoenicia, Iroquois replaced by Cree, Shoshone replaced by Mapuche, Assyria replaced by Sumeria, Songhai replaced by Mali)
Overall, that's only three civs in the entire history of the franchise that were outright dropped (and not replaced) from one installment to the next. And even that is maybe being overly critical, as Morocco was incorporated into Mali and the Ottomans, and a VI version of Venice's playstyle was effectively assigned to Portugal. So neither was totally lost.