Many projects also deliberately avoid publishing a version 1.0. For example, the static website generator Hugo continued with 0.100.0 after version 0.99.1. The current version is 0.123.3.
This is mostly due to the fact that some users have the wrong expectations. For example, that version 1.0 is as good as error-free and fully developed.
How long has it been the de facto default greeter for KDE Plasma?
Since Plasma 5 if I’m not mistaken. Before that, KDM was the standard.