I'm not familiar with any general requirements for thermal transfer labels for archived documents. Regardless of the printing longevity, label adhesive tends to deteriorate over time and would be a weak link. If the adhesive doesn't fail so the label falls off, it can bleed into the paper and discolor the document. I would think attaching a printed page rather than applying labels would be more permanent.
If you do need labels, explore what is available in labels designed for archival purposes. These would require both the paper and adhesive to not deteriorate for a very long time, and are likely to have a surface finish or coating optimized for the type of printing.
If you're talking about archiving documents, there's an implied caveat that the documents are properly stored. This means "normal" temperatures and humidity, etc. If archived documents are stored in conditions like an unconditioned warehouse in a hot, humidity climate, or get soaked with water during a flood, "archival quality" won't be enough. All of the description that follows assumes proper storage conditions.
In terms of printing if you need labels, I would expect archival inkjet printing to be the longest lasting. Pigment inks don't fade, and dye-based inks can be made to last many decades without deterioration. Typically, archival inkjet printing focuses on both the ink and the paper. Long lasting ink won't do you much good if the paper darkens and becomes brittle. Beyond that, archival inkjet papers also focus on the surface the ink needs to bond with.
Laser printing doesn't fade, but it is bonded just to the surface. Very old laser printing can sometimes flake off. The toner can also stick to the paper of the previous sheet pressed against the printed page in the stack, and get pulled off when you separate the sheets.
Thermal transfer and thermal printing are very different. Thermal transfer uses a ribbon with a layer of printable material. Heat applied to the ribbon melts the printable material and it sticks to the paper. This won't fade, but depending on the type of ribbon, it can flake off, or stick to another page, similar to laser toner.
Thermal printing uses heat to affect chemicals embedded in the paper and turn them dark. Depending on the chemicals and storage conditions, the dark portion can be fairly long-lasting, although it is probably the least long-lasting of the printing methods and generally not considered archival duration. Some of the chemicals also have a tendency to turn dark on their own eventually, and some are sensitive to other forms of heat, like friction; the cumulative effects over time can darken the unprinted areas.
The temperatures used for thermal transfer and direct thermal printing aren't necessarily the same. It might be possible to use thermal paper in a thermal transfer printer without the ribbon, but the results likely wouldn't be optimal. The paper would also be more abrasive than the thermal transfer ribbon, and the thermal printhead might not be designed to last with that wear and tear. You might also get buildup of materials from the paper surface on the printhead that would affect print quality.
If you can provide more detail on the requirements you've been presented with, we may be able to make a more specific recommendation.
