This is by design, afaik - 20 users is the limit for a chat to have the urgent messaging functionality. You can always submit a feature request to have this limit changed, do so over at the Feedback portal: https://feedbackportal.microsoft.com/feedback/forum/ad198462-1c1c-ec11-b6e7-0022481f8472
Urgent alert button missing on group chat
There was talk about this in 2020 where in group chats the ! button would disappear from the lower chat bar, though could be found along the top bar when in format mode. My issue is, is that it seems after 20 people are added to a group chat, the urgent option is removed. This is causing issues since we have chat groups that have more then 20 people, and there is a need for the Urgent option, and would be nice if the important/urgent button would stay down at the bottom even when there is more then 20 people in a chat group. Is there any talk of having the current set up changed to not have a max number, or is this not going to change?