Hello EmmaneulFaure,
Manganin does not have a perfect zero slope TCR at the cardinal temperature, at the very least not consistently, the hyperbolic TCR curve of Manganin is inherent in the alloy, it is 'considered' relatively 'flat' for about ±5°C from the cardinal reference before the TCR starts rapidly increasing. Zeranin, a relatively new derivative, increases the relatively flat portion of the TCR curve to about ±10°C from the cardinal before taking the same serious change in TCR rate. I believe there is one or two other derivatives which make small tweaks to the 'flat' portion of the curve and extends the upper range to about 65°C, this not to say the TCR remains as low as at the cardinal point, this extension was made by changing the mix of metals in the alloy slightly, the overall characteristics are still basically the same.
Low ohm resistors and shunts are still made with the Manganin family although it is not generally used for standards any longer being replaced by the Evanohm family alloys, they not only have a very linear TCR but their stability is better and are tough to boot. Manganin cannot be operated at elevated temperatures, not much above 65°C because they anneal at very low temperatures and that changes them permanently.
Manganin's lack of use is primarily because they have been replaced by superior alloys and better technology in using those alloys. Manganin was easy to use because it could be soldered (carefully), Evanohm cannot be soldered, it must be welded.
No matter what the alloy used for a low TCR / high stability resistor, the processing of the resistor demands very careful controls and depending on the technology used, those processes are considerably different. My processes are very different from Vishay's as there are different variables involved. Another consideration is that current metallurgy cannot control the batch mix of metals tight enough to provide consistent characteristics batch to batch, the fact that we can produce very low TCRs with very long low stabilities these days can be attributed to some 'cherry picking' of the alloy characteristics as every batch of alloy is different. While the resistor industry can push the alloy providers for ever tighter mix controls there is a limit as to how tight they can get it for a reasonable price. My most expensive wire runs over $18,000 a pound which depends on the wire size, I have no control over that aspect. I have a very limited number of alloys which can meet my wire specifications, outside of those, there are none.
Just remember, whether it is a PWW or a metal film/foil resistor, your TCR limits is going to cost more and more as you demand zero TCR because those very low TCRs are in short supply, we really can't create them at will unfortunately.