I tried to add stuck projects to my block agenda, but I did not find the relevant
keyword. Then I googled and it found this discussion which mentions the stuck 
keyword:

https://old.reddit.com/r/orgmode/comments/96z3el/display_list_of_stuck_projects_in_the_agenda/e45yx8p/

Shouldn't this 'stuck' keyword appear on the Block Agenda manual page along with other ones?

https://orgmode.org/manual/Block-agenda.html