“admin/people” filters not available anymore
I would check:


Is the views module still enabled?
Is this specific view enabled?
Did the configuration get changed for the view that exists at admin/people?


For #1, go to the modules page and scroll down to views and confirm it is enabled.

For #2, I noticed that I was able to "Disable" the user_admin_people view and then I get a screen at admin/people that looks exactly like yours. The resolution to that problem is to go to /admin/structure/views/ and "Enable" the user_admin_people view.

For #3, you could install a fresh version of Drupal 8.7.3 and compare the configurations of the view to find if it's wrong.