There is still some virtual private servers that rely on gcc 6.x series and Ubuntu 18.04 as gcc 7.x series is known to cause issues when running inside qemu based virtual machine on some Intel Xeon processors. Symptoms included for example corrupted pointers (NULL pointer is -8 instead of 0).
As some servers are meant to run 24/7 uninterrupted and have uptime of several months up to about 5 years, switching to newer Ubuntu release that might not even compile or run required software correctly has not been even an option.
Back when some servers migrated from Ubuntu 16.04 to Ubuntu 18.04, it initially caused instability issues and crashes on essential software needed by some financial organisations. Due to some dependencies having changes that were not backwards compatible, code also had to be adapted which caused unneeded extension to interruption needed just for upgrading the operating system and essential packages.