Rocket.Chat/app/ui-message/client/messageBox/messageBoxReadOnly.html

<template name="messageBoxReadOnly" args="rid, isSubscribed">
	{{#if customTemplate}}
		{{> Template.dynamic template=customTemplate }}
	{{else}}
		<span>
			{{_ "room_is_read_only"}}
		</span>
		{{#unless isSubscribed}}
			<button class="js-join rc-button rc-button--primary rc-message-box__join-button">
				{{_ "join"}}
			</button>
		{{/unless}}
	{{/if}}
</template>