Li Shenron
Legend
Uh...that's not how I remember it. Specialties were originally something everyone got, at level 1, which had various effects.
I still have the playtest docs although not here right now, but I googled for them and found the "Specialties and Feats" chapter, although I am not sure from playtest packet it belonged to.
Here Specialties were just sets of 4 pre-selected feats, and nothing else in addition.
Three options were given: (a) pick a specialty as-is, (b) customize a specialty (i.e. take other feats in place of those mentioned by the specialty), (c) ignore specialties and choose feats a'la carte.
Admittedly, option (c) is mentioned to be "at your DM's permission". However option (b) apparently doesn't require DM's permission, but is de facto equivalent to (c) since it doesn't have any restriction except the feats prerequsities (which applies to all 3 cases anyway).
IMO specialties were removed when WotC decided to drop the idea that all classes should get the same number of feats (or stats increases). At some point, they decided it was fine for Fighters to get more feats than Wizards and so on. They could have strived for keeping the same rate for all classes, but dropped the idea [and to be honest... IMHO the true underlying reason was that it was just too much of an effort to design the same amount of level-based class features for all 12 classes; not wanting to have any 'blank levels', they filled the blanks with feats / stat boosts]
Once the number of feats was not going to be the same for all characters, then specialties became more difficult to present. Because originally you just chose one specialty, but now many classes were going to have more than 4 feats, so everybody would end up with one-and-a-half specialty, or equivalently one specialty + some loose feats.
Also notice, that strict specialties weren't directly compatible with the (promoted to very important) idea that feats are optional, and you can always take stat boosts instead.
At this point, you would have to write a lot of text about alternatives to taking specialties as-is, so it was just easier to default to option (c) i.e. say everybody picks feats one by one.
"Were specialties ever considered as a means of implementing multiclassing during the development process?
Yes this was and is still (at least partially) true. That's why we have feats which grant spellcasting. I wish they had kept those feats which granted higher-level spellcasting, but they removed them apparently. Maybe this is only because they will resurface later in the development of 5e, when it will be clearer how to balance them.