A slotted line is a fundamental Standard of Impedance. It relates impedance
measurements to the absolute standard of length, the International Metre!

Its weakest feature is the probe mechanism itself. Its electrical presence
is unpredictable. Incalculable. Uncertainty can only be minimised.

Or am I 60 years out of date?

Does anyone know what is the uncertainty in measuring an impedance of, say,
around 100 ohms at 10 MHz, as may be claimed by a National Measurement
Standards Laboratory.

What may be claimed by an instrument manufacturer in the sales blurb is
another matter.
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Reg, G4FGQ