According to the Ubuntu release schedule, Canonical announces new Ubuntu version approximately one month after the new GNOME release.
However, for the next Ubuntu 21.04 “Hirsute Hippo,” which is scheduled to be released on April 22, 2021, Canonical has decided to change its normal plan.
Yes, going by tradition, Ubuntu 21.04 should ship GNOME 40 desktop, which will arrive in March 2021, one month before new Ubuntu release.
But now, according to a new announcement by Ubuntu’s Sebastien Bacher, Hirsute Hippo will stick to GNOME 3.38 with GTK3, instead of GNOME 40 with GTK4.
According to Sebastien Bacher, the reason behind this is the stability of a new GTK4 toolkit, a major shell design overhaul in GNOME 40, and its impact on GNOME extensions and Yaru GTK theme, which Ubuntu includes by default.
Owing to such reasons, this Ubuntu release cycle will not feature GNOME 40 and GTK4 updates.
Nonetheless, Ubuntu 21.04 will definitely ship other changes like private home directories by default.
Besides Ubuntu, the Fedora team is planning to feature the latest upstream GNOME release, GNOME 40 in the upcoming Fedora 34, which will also arrive in April this year.
At last, if you want to try the development version of Ubuntu 21.04 Hirsute Hippo before its stable release, you can download its daily updated ISO image from here.