Currently, there appears to be no way to minimize all windows in Chrome browser. Instead, users need to click (or use the shortcut) to minimize each window individually.

While it's not uncommon for users (particularly Chrome OS users) to have a large number of windows open, making this process cumbersome, particularly after software updates where windows are restored and opened automatically.

Threads on Chromebook Product Forum:
(1) "How to Minimize and Hide --All-- Windows in Chromebook OS?"
https://productforums.google.com/forum/#!topic/chromebook-central/nGaekoC8odE;context-place=topicsearchin/chromebook-central/minimize$20all

(2) "Show desktop shortcut (minimize all)?"
https://productforums.google.com/forum/#!msg/chromebook-central/C4Kydk3KBeE/ovifqQlF5SAJ

Sample User Feedback reports requesting ability to minimize all on Chromebooks:
(1) https://listnr.corp.google.com/report/85612792563
(2) https://listnr.corp.google.com/report/85588480795
(3) https://listnr.corp.google.com/report/85443913745
(4) https://listnr.corp.google.com/report/85324049297
(5) https://listnr.corp.google.com/report/85234256826
(6) https://listnr.corp.google.com/report/85230094403
(7) https://listnr.corp.google.com/report/85169536643