wrote

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.
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Yes, and in typical configuration it is an electrical 1/4-wave coaxial
section connected in parallel with both conductors of the main transmission
line. It does not terminate the main transmission line, so this
application/example is not very relevant to the "pin through the coax"
question of the OP. And it would not result in a dead short at the carrier
frequency, no matter where it is located in the output system. Some FM
broadcast antennas also include them to supply a DC short from inner to
outer conductors of the antenna coax to provide some protection from
lightning.

A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast
band. The need there is to add a deep notch at stations whose 2nd harmonic
falls in the broadcast band. This stub is used to add to the attenuation of
the already compliant 2nd harmonic level coming out of the tx, but which,
without the stub can be heard on broadcast receivers within a short distance
from the broadcast antenna site. WJR (760 kHz) is one station using this
technique.

RF (WJR engr in mid-1960s)