Plural form always for paid tiers

app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml

-= form.label :assignee_id, issuable.assignees.size > 1 ? _('Assignee').pluralize : _('Assignee'), class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
+= form.label :assignee_id, issuable.allows_multiple_assignees? ? _('Assignees') : _('Assignee'), class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
 .col-sm-10{ class: ("col-md-8" if has_due_date) }
   .issuable-form-select-holder.selectbox
     - issuable.assignees.each do |assignee|

app/views/shared/issuable/form/_metadata_issuable_reviewer.html.haml

-= form.label :reviewer_id, issuable.reviewers.size > 1 ? _('Reviewer').pluralize : _('Reviewer'), class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
+= form.label :reviewer_id, issuable.allows_multiple_reviewers? ? _('Reviewers') : _('Reviewer'), class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
 .col-sm-10{ class: ("col-md-8" if has_due_date) }
   .issuable-form-select-holder.selectbox
     - issuable.reviewers.each do |reviewer|

locale/gitlab.pot

@@ -23405,6 +23405,9 @@ msgstr[1] ""
 msgid "Reviewer(s)"
 msgstr ""
 
+msgid "Reviewers"
+msgstr ""
+
 msgid "Reviewing"
 msgstr ""
 

spec/features/issues/user_edits_issue_spec.rb

@@ -387,4 +387,4 @@ RSpec.describe "Issues > User edits issue", :js do
       end
     end
   end
-end
+end
\ No newline at end of file