More than a decade ago, Greg Kroah-Hartman started offering some Linux versions with significant support. Linux 2.6.32 was designated as a "long term" support release, even though the term was just about 2 years.
Fast forward to today and "longterm" releases have actually become long term, i.e. they provide 6 years of support. That is, until now. While I have no doubt those releases will keep being marketed as having "long-term support", that support is actually being cut back to just 2 years.
The first argument provided ("There's really no point to maintaining it for that long because people are not using them.") is doubtful to say the least, as the most popular GNU/Linux vendor still supports Red Hat Enterprise Linux 7's Linux 3.10, older than the oldest supported vanilla Linux. The second one ("Linux code maintainers are burning out") though, is certainly true. Indeed, Coase’s Penguin warned about the challenge of integration even before Linux 2.6 released:
And unfortunately, our global governance certainly hasn't gotten us any closer to a solution for that actually long-term problem.