Work on Debian 10 is under way already even though we won’t see it ready for mainstream consumption until 2019. One of the latest changes discovered in Debian 10 is that it drops X11 for Wayland. Debian is considered quite conservative in terms of how quickly it adopts new technologies in order to provide a stable experience and therefore will still continue to offer X11 as an alternative.
Adoption of Wayland hasn’t exactly been quick. It started to gain attention from around 2010 by various news outlets that focus on Linux developments but it has only been in the last couple of years that we’ve actually seen it adopted by Linux distributions. Fedora, a distribution which is fairly cutting edge, has already switched to Wayland while Canonical plans to include it with Ubuntu 17.10 when it ships with GNOME. The main reason that Ubuntu has taken so long to switch is because it was only recently when it decided it was going to fall back into the fold and abandon its Unity and Mir projects.
The X Window System originated at MIT in 1984 and reached its current version 11 in 1987. As you can probably guess, it’s outdated by today’s standards and therefore Wayland has come to replace it and offers more flexibility and better performance.