I realized that the green spectrum could theoretically enhance the efficiency of NaSc metal halide bulbs. So why wasn't it ever used? I get it; the CRI would probably be worse, but it would still be fine for outdoor lighting. However, Thallium was never used in NaSc lamps.

Attached below is what the spectrum of a NaScTI lamp could look like.

I've an Aliexpress 5000K MH lamp which have Na-Sc-Dy-Tl.

One of the very first metal halide lamps from Westinghouse was actually thallium dosed.  It was marketed as a super high output mercury lamp, but was unpopular because of the terrible colour rendering and was quickly superseded.

Like James said... In fact, many NaSc  MH lamps have thallium added, but with some extra sodium and indium too to bring the color back to white. So they are somewhat in-between pure NaSc chemistry and tri-band In-Tl-Na chemistry, hitting CRI index of about 70-75.

Adding some thallium to NaSc lamp,  without extra sodium at least, will sure increase the efficiency, as more light will be radiated at green where the eye is most sensitive, but will give the lamp a really hateful green tint with rather pronounced metamerism.