Savage,
Jump portals did exist in Traveller as early as CT, but the tech level required to create and use them is well above Imperial technologies, which is why you don't see them in the OTU around important worlds like Capital. (IYTU, it could work out well, and would make for some interesting gaming, though. The rest of this is just some OTU stuff.)
According to a CT-era product, Grand Census specifically, on pg 37 under Space Transportation Technology, the Starship-sized Matter Transmission Portal (or jump portal) is the benchmark used to evaluate a world as having TL21. Later products, such as MegaTraveller's Referee's Companion (pg 29), also confirm the presence of such technology as a sign of TL21.
Portals are also mentioned in Adventure 12, Secret of the Ancients, a CT adventure by GDW released a few years before Grand Census, but the TL needed to repair a broken portal in that adventure is TL25.
Jump Portal technology is likely to be developed simultaneous with the Jump Projector weapon, as both technologies have a hallmark of no longer requiring the physical presence of a jump grid/jump drive/lanthanum coils on the target in order to send the target into jump. That's also TL21.
I haven't read the two T4 adventures mentioned above, so I'm not certain how they explain the presence of these portals. If it conflicts with CT canon, though, I'm not likely to accept it as something that establishes canon for MTU. (Now, for an ATU, that's cool.) But that's just me. YMMV.
Hope this helps,
Flynn