I have custom fields on my wordpress installation but after theme update, they disappeared from admin. They still show the content on front-end, but I can't add new content from admin. I already checked if it was unchecked in "screen-options", but it isn't there ...
I tried to deactivate every plugin and see if anything changed, but no.
I don't know what version you had, I am using Wordpress 5.2+. I had the same problem, I can't find the checkbox of Custom Fields in the Screen Options as it suppose to be but later on I found it in the Options of adding or editing post. Go to Posts > Add New > click on 3 vertical Dots on the top right conner > click on Options > once the options dialog box popped up you will see Custom Fields checkbox under Advance Panels.