[mrob]

Who says you have to use an analog PLL decoder? FM audio tape is clearly capable of storing DC because you can store a constant sine wave at a different frequency to your carrier frequency. Maybe you'll decode it with software defined radio software.

[sobriquet9]

Even when you decode with an SDR, you still need to adjust the receiver frequency to match that of the transmitter, as reference oscillators might have slightly different frequencies that can also slowly shift, e.g., due to changes in temperature. In case of a tape recorder, tape speed during recording and playback can also be slightly different due to mechanical factors.

[mrob]

Sync everything to GPS. Record a harmonic of the carrier on the same tape, or just record the carrier itself on a parallel track. Have the SDR use this reference tone to undo any tape speed fluctuation artifacts. You can bring the deviation from DC so low that your comment no longer has any relevance to tape machines and just becomes a pedantic comment that mathematically perfect DC doesn't exist in physical reality.

[sobriquet9]

Tape machines predate GPS.

A metaphor should be referring to something common, not something that can be made in theory to fit a different set of requirements, but does not really exist.