It recently occurred to me that while I've seen government regulations imposed on T12 lamps, I've seen nothing of the sort for T10s. Has anyone else heard of such legislation? T10 lamps are certainly expensive and wouldn't make sense for a large-scale mainstream installation compared to T8s, but as long as they are still in production, our old fixtures should be safe. The only drawback I've found with T10s besides price is that they tend to flicker more on certain ballasts, but it usually goes away after a little while. The important thing is that all the ones I've seen are full power, so they're safe for the older ballasts.

I am fairly sure that T10 lamps are not included in the ban. The T10 lamps are efficient enough that they would probably meet the current T12 standards anyway. Most of the new F40T10s are high CRI and high lumens, and they are more efficient than 700 series F32T8s!

Are F40T10's completely safe for an old school full power ballast? Wouldn't the thinner size alter the arc voltage?

I've run T10s on old preheat ballasts and they seemed to operate normally. That's all I can say about that.

Measure the arc voltage: If it is the same ~100V as the T12 (+/-10% tolerance is no problem), the ballast would be safe...

While T10s are indeed thinner, I would assume the gas fill is appropriately altered to offset that. My own F20T10 has the same arc voltage as an F20T12. If the process used to lower the arc voltage is the same as that used to make, for example, a 34 watt F40T12, that would explain the flickering when cold.

T10 lampS cause the ballast to draw slightly more line current,  the wattage was usually 3-4 watts more using 2 F40t10 versus F40t12 lamps
That could in theory cause the ballast to run slightly hotter maybe cooler? Its a tiny difference anyways...

More current equals a hotter ballast. Assuming you're on an HPF ballast, if wattage is going up then I think that means the arc voltage is slightly higher than an F40T12. If you tried it on an LPF ballast, current would be lower than with an F40T12.

But I think what you're observing is just normal lamp-to-lamp arc voltage variation. For example, my two 34 watt lamps differ by 10 volts, which causes a significant difference in current draw.

Where is the best place to buy 4 foot T10 lamps? I've never seen a T10 in the wild.

1000bulbs and elightbulbs both have them.

Do you guys really for see a day when we can't get any more T12 lamps at all? Seems hard to imagine.

I think that either production will be banned or manufacturers will simply stop making them once demand goes away. The latter will probably not happen for a very long time.

As long as a ban isn't enforced, I imagine all T12s could be manufactured for at least another decade. And I can't imagine a ban occurring on lamps smaller than the F40T12 at all. Those lamps (F15T12, F20T12, etc.) would eventually succumb simply to lack of demand, although I have no idea how long that would take. I rarely see small T12s in use any more, yet every store I go to is still fully stocked in every type. Same with the F15T8. I still see even F26"T8 lamps for sale, and those were only ever used in stoves from 30 years ago! As for the F30T8, I've never seen a fixture or lamp in my entire life, and I've never seen them for sale in stores, yet I've been told at least GE still makes them.

Wow, you've never seen an F30T8 lamp? I've seen quite a few in stores. I'm pretty sure that the Big Three as well as Halco still make them, and if they don't, they're still available. As for F40T12s, they could be around for another 50 years if they're not banned. Just look at the F90T17...there hasn't been a fixture made for one since the 1950s and GE only recently stopped production. Not only are F40T12 ballasts in production today, you can even still buy F40T12 fixtures brand new! Same goes for the F96T12. Fortunately for us enthusiasts, it seems that T12s are indeed losing popularity, so there should be less of a reason to ban them completely. It still makes me happy to see decrepit T12 installations relamped and made nice again. A small convenience store not too far from me recently repaired their lighting. They replaced the T12 Slimlines across the front of the store with what look like F32T8s from a distance, but the six 1950s-60s 2x96 Slimline fixtures over the gas pumps (the old kind with the mechanical dials) were finally relamped after years of being poorly maintained with what look to be CWS Altos. I assume they also had to replace some of the ballasts...these lights had been neglected for many years.

I remember an old gas station in Atascadero, CA that had been turned into a used-car lot that had some really neglected strip fixtures.  They were all out, many lampless with a few having what I now know to be yellowed-ends tube guards.  Maybe one was working actually.  Then one light at each corner got daylight Altos, then sometime in the late 2000s or early 2010s was totallu redone to T8 but all fixtures worked last I saw.  They were painted over strips too but with new lamps.  Whenever I see these retro jobs I always wonder if the original ballast is still in there.
This was all under the old gas station canopy BTW...