The most likely reason is that Pepsi went to court over them
Essentially, Pepsi tried to register a community design (kind of like a trademark, but for shapes, lines, textures, colors, etc. that are associated with a product), but an organization named Grupo Promer had registered something similar first. The court cases were over whether or not Pepsi's pogs were distinctive enough to be granted their own protection. In other words, could a single company have exclusive rights to a plastic or metal disc included in food packaging? How distinctive would a competitor's have to be before they were unique enough?
From this summary of the court cases:
On 9 September 2003, PepsiCo filed an application with OHIM (Office for Harmonisation in the Internal Market) for the registration of a Community Design pursuant to the CD Regulation. Grupo Promer then claimed priority for its Spanish design No 157156, which had been filed on 23 July 2003 and which application was published on 16 November 2003. Both registrations concern the design for "pogs" or "tazos": promotional items in the form of plastic or metal discs that are often included in food packaging and are intended mainly for children.
Grupo Promer started an invalidation action before OHIM against PepsiCo's design application, arguing (i) that PepsiCo's design lacked novelty and individual character (on the basis of Article 25(1)(b)) because of Grupo's Promer's earlier Community Design for metal promotional plate(s) for games; and (ii) that PepsiCo's design was "in conflict" with Grupo Promer's prior design (pursuant to Article 25(1)(d)).
As best I can tell, Pepsi wound up losing this case because there wasn't enough distinctiveness in their pogs compared to those of Grupo Promer. The last mention I can find is this from 2011:
PepsiCo then appealed to the ECJ on the grounds that the General Court had incorrectly interpreted and applied Article 25(1)(d) CD Regulation (conflict with prior design). The Advocate General urged that the appeal be dismissed, noting that a significant number of the grounds of appeal actually concerned findings of fact, and these are not suitable for appeal to the ECJ.
Since there haven't been any further mentions of this case, I'm going to assume that the appeal was dismissed.
I should note that this is a very plausible explanation for the term "forbidden pogs", but I could not find the term in any of the legal documents or summaries. There does not appear any documented use of origin of the term.