The question in the link is actually about the length of 300 feet (really 100 meters, or 328 feet), but the answer discusses 300 meters, and I am unaware of any ethernet standard that had a limit of 300 meters.
For the common ethernet standards that run on UTP (10Base-T, 100Base-TX, 1000Base-T, and 10GBase-T), the distance limitations are for 100 meters, assuming up to 90 meters of solid-core horizontal cable (permanent link), and a maximum of 10 meters of stranded cable (split between both ends) for a total channel length of 100 meters.
The problem arises as you increase the frequency of the signal to get faster speeds. To do that, you must change the cable to continue to meet the 100 meter standard. ANSI/TIA/EIA has recognized cable categories (Category 5e, Category-6, and Category-6a; other cable categories have come and gone), and has a set of tests* that must be matched for each category to be certified for that category.
Understand that a long wire is really an antenna, both radiating and receiving extraneous electromagnetic signals. If the noise on the cable becomes too much, the signal cannot be distinguished from the noise. Twisting the differential pairs of wires within the cable gives some immunity to noise, but there are four pairs of wires within the cable sheath, and they each will send out and pick up signals from the other pairs in the sheath, along the entire cable length, sending the signals in both directions, and to and from adjacent cables. This is called crosstalk. The longer the cable, and the higher the frequency of the signal, the harder it is to control the noise on the wire. Cable vendors will take steps, e.g. more twists per inch, bigger wire gauge, or even adding a shield around the individual pairs, to try to meet the new requirements for a new cable category.
If the cable passes all tests after being properly installed, it is guaranteed to work for up to 100 meters. That is not to say that some people do not fudge that and install longer lengths, which may or may not actually work (possibly with lots of retransmissions). Also, poor installation could severely limit the length of cable that will work, but the cable would not pass the test suite for the category. Small things in the installation, e.g. untwisting a pair slightly too far on termination, can cause a cable to fail the category test suite.
*The primary tests are:
Wire Map - Checks for proper pin to pin termination,
and for each of the 8 conductors the wire
map checks for: Continuity to the far end, Shorts between any two or more conductors, Reversed Pairs, Split Pairs, Transposed Pairs, Any other miswiring.
Length - The physical length of the cable is the actual length derived by measurement of the cable(s) between the two end points. The electrical length is the length derived from the propagation delay of the signal and depends on the construction of the cable. The maximum physical length of the horizontal cable (permanent link) one end of the cable to the other is 90 meters. The maximum length of the channel model is 100 meters.
Insertion Loss - Insertion loss is the loss derived from inserting a device into a transmission line. The insertion loss for both the permanent link and the channel models are the total insertion losses of all the components.
Near End Cross Talk (NEXT) - Pair to pair NEXT loss is the measurement of
signal coupling from one pair to another. The result is based on the worst pair to pair measurement.
Power Sum Near End Cross Talk (PSNEXT) - Power sum NEXT takes into account the statistical crosstalk between all pairs while energized. This is a calculated amount derived by adding up the crosstalk results between all pair combinations.
Equal Level Far End Cross Talk (ELFEXT) - FEXT is the unwanted coupling of a signal induced by a transmitter at the near end, measured on the disturbed pair at the far end. ELFEXT is the same measurement of FEXT, less the effect of attenuation.
Power Sum Equal Level Far End Crosstalk (PSELFEXT) - As in Power Sum NEXT, these are computed values based on the sum of all the possible pair combinations under the respective tests.
Return Loss - Return loss is the value of energy reflected by impedance variations when devices are inserted into the cabling system.
Propagation Delay - Is the time it takes the signal to travel from one end of the cable/system to the other. The maximum channel propagation delay is 555ns (nanoseconds) and for the link it is 498 ns, both measured at 10 MHz.
- Delay Skew - Delay skew is the signalling delay difference in time (nanoseconds) between the fastest pair and the slowest pair. The maximum channel delay skew is 50 ns, and in the permanent link it is 44 ns.
Any tests that are out of specification will fail the test, and the condition must be corrected and the test suite performed again until the cable passes or is replaced.
You can also permanently damage a cable by exceeding the pulling tension or minimum bend radius when installing the cable run.
Also, splices, taps, couplers, etc., are not allowed in a cable run.
There is a newly recognized category (Category-8), but it is shielded cable (vs. the UTP cabling for the other categories), and the distance limitation is 30 meters (24 meters for the solid-core horizontal cable, and a total of 6 meters of stranded cable, split between both ends). This category was recognized to support 25GBase-T and 40GBaset-T.