new to GMs and just read up about them. Are they still needed if the car has a front and rear suspension kit installed like Speedtech or Detriot Speed? If so, are there new-tech aftermarket options out there? Seems easy enough to design an air spring for the corners which would weigh significantly less.

My opinion, I believe the "systems" correct two different issues.

Suspensions correct sway, bounce, and dipping from braking and acceleration. The "cocktail shakers" are vibration dampeners. Early Corvair and 1st Gen GM F-body convertibles were susceptible to vibrations caused by the flexible body so they added the vibration dampeners. If I recall, the vibration dampeners were configured to absorb the specific vibration harmonic of those bodies.

Short answer, yes they are needed.

I drove mine without them in and experienced pretty healthy "cowl shake"

As Jim said, they absorb vibration and are not "springs".

They contain transmission fluid with weight suspended between 2 springs.

Jim do yo have the link to that article I found a while back?

Good idea. Here it is:

Mine were long gone when I got my car 17 years ago. I’ve never missed them

Thanks Jim for the article. I hope to get delivery of my 69 in a week, but know the rears are missing the shakers and also likely the fronts as well, as the car has a Speedtech subframe with an LS swap done a few years back and probably were removed back then. I'm still not sure why anyone would remove them purposefully, unless they are an obstruction. Like relocating a battery to the trunk, the shakers aren't taking space otherwise. I also cam across this article for gen one Camaro verts which was help explaining the canisters. I guess, I'll need to drive the car a bit to determine whether I'll need to get them replaced. Hopefully not, as they seem to go for a premium in the secondary market and I guess no one makes an aftermarket version at the moment.

http://www.chevy-camaro.com/faq/chevy-camaro-convertible-cocktail-shakers.shtm#:~:text=A.,lack%20of%20the%20roof%20structure

Adding to the conversation. I just reinstalled my cocktail shakers while putting my bird back together. They shakers bolt to about 5 different spots including the wheel wells, radiator support, and other spots to tie in all these locations. Adds structural safety if you were to get hit in the front of the car. They also hold the front fender in place at the top where the rubber bumper sits just under the fender.