But if the Star Empire is in abrogation of the treaty, doesn't that make it void?
As I understand it, treaties can only be voided if both sides abrogate (that is, repeal) them legally, because they assume the status of domestic law once they have been ratified.
Thus, even if the Romulan Star Empire repealed the Treaty of Algeron, it would still be binding upon the Federation, just like any piece of domestic Federation law, until the Federation repealed it.
Personally, I think it would be irrational for the Federation not
to repeal the Treaty in the post-Typhon Pact landscape; the RSE is in no position to attack the Federation unilaterally (which was the original reason the Federation agreed to ban cloaks in the Treaty: to reduce tensions with the Romulans, as established in The Lost Era: Serpents Among the Ruins
), and the rest of the Pact does not have a Treaty with the Federation banning the cloaking device. Meanwhile, the Federation is allied with the Imperial Romulan State, which opens up new avenues of trade and scientific exchange for developing new cloaking technologies -- and they
aren't likely to attack the UFP for developing cloaking technologies.
Meanwhile, the very existence of the Pact means that Starfleet Intelligence is going to need to step up its espionage missions, and Starfleet is going to need to be able to keep up with its cloak-capable Khitomer Accord partners (especially if the other Khitomer members, like the Cardassians and Ferengi, decide they want cloaks, too). It would be irrational for the Federation not
to repeal the Treaty of Algeron and allow Starfleet Intelligence to operate a fleet of cloak-capable ships for use in reconnaissance and espionage missions, particularly agent insertions and extractions.